Handling Pre cat lacquer

[ivan]

It's not easy for the amateur to get helpful info on sprayable finishes, even if you ask the supplier (he seems to think you'll know already). I've yet to find a book that does more than provide a tantalising overview. Years ago a mate taught me enough to put down a passable coat of cellulose on a dented wing. My attempts to spray furniture are built on that simple beginning, but now at lower pressure (hvlp)

My last was some bedroom items that had to be finished in matt white. I chose pre cat (over pre cat primer) because I thought it would be easier to handle (no separate catalyst). A combination of poor technique and a desire to get a bit more on the routed edge of some mdf may be the cause some hair line cracking in the finish? Certainly it seemed to occur where the lacquer was thickest.

Do you have to be careful how many coats of pre cat you lay down? What about catalised lacquer? Does this sort of info ever appear in a book? Thanks in anticipation

 

[Scrit]

I've yet to find a book that does more than provide a tantalising overview.

Then you're in luck! Try getting hold of Andy Charron's "Spray Finishing" (Taunton Press, ISBN 1-56158-114-3). It's probably the only book on furniture spray finishing written by a professional furniture sprayer. Some of the terms are a bit American, but it's a heck of a good reference. Includes stuff about HVLP and water-based, too.

A combination of poor technique and a desire to get a bit more on the routed edge of some mdf may be the cause some hair line cracking in the finish? Certainly it seemed to occur where the lacquer was thickest.

How did you seal the MDF? Was the MDF raw? MDF edges frequently need 3 to 4 coats of a primer/sealer before you start spraying. Was the primer compatible with the top coat? Did you put the scoats on thickly? Were the work piece, the lacquer and the booth all at a reasonable temperature (i.e. above 15 degrees)? Lots of questions before I can answer, I'm afraid.

Do you have to be careful how many coats of pre cat you lay down? What about catalised lacquer?

No, not really [*see below], but you do have to ensure that you spray second/subsequent coats within the open window and make your coats thin - just a bit more than leaving "orange peel". If your booth temperature is right then it will flow niceley before it starts to go off.. Some finishes take 10 minutes to flash off before you can recoat, then need to be recoated within a set period (normally an hour) or left for 24 to 48 hours BEFORE applying the next coat. Depends on the finish, really. Personally I'd stick to pre-cat to start with. With 2K (2-pack or acid catalysed) finishes the refinish time windows become much tighter and the stuff has also got a limited pot life (the time before it sets inside your gun and cream-crackers it) of somewhere between 1 hour (worst I've ever dealt with and 4 hours). Some 2Ks are toxic and require special bled-air breathers because they give off unstable cyanic compounds as a by product if the curing process - these compounds break down quickly (minutes) but you still don't want to breath them. They can be differentiated by the pear drops smell/taste associated with some cyanic compounds. Precat tends to be something like a day or more pot life and some can even be left in the pot overnight providing it is removed from the gun and cling filmed over.

Hope that helps

Scrit

NOTE: Please read my next post, further down, together with what Sgian Dubh has written. The AMOUNT of lacquer you deliver to a given area is important because if you put it on too thickly (or too many coats in one session which amounts to the same thing) you can get all sorts of defects occurring including solvent trapping, sagging, runs and edge cracking. Note appended for purposes of correctness.

 

[ivan]

Thanks, Scrit, I'll look out for the book.

Mdf raw edges not treated with a sealer as such; they had 2 coats pre cat white primer, then rubbed down till quite a bit of mdf grinning through. Then whole job primed, + 2 coats matt white pre cat finish from same maker. Edges probably got the three coats twice (when each side sprayed) and also some tricky bits and corners. (my poor spray technique) These heavier coated areas are where the hairline cracking ocurred, 2 or 3 days after the job was done. Before I saw the cracking I was quite pleased with the results!

I didn't know there was an ideal coating window for pre cat - I thought the finish would dissolve into the next coat like cellulose, as the cure takes several days, (not hours like 2 pack).

In practice there was about 2 hours between coats on the smaller items. The largest, a run of built in wardrobes, had to be sprayed in situ with cross ventilation from an industrial fan in the next room. With the air down as far as possible to limit overspray that spray job took me a couple of days. The temp. was in the mid 60's, the lacquer well thinned to atomise with the reduced air, so a really thick a coat would have run/sagged.

PS apart from primer or sanding sealer, what do you use on mdf raw edges?

 

[LyNx]

PS apart from primer or sanding sealer, what do you use on mdf raw edges?

Watered down PVA is ok

Andy

 

[Scrit]

I didn't know there was an ideal coating window for pre cat - I thought the finish would dissolve into the next coat like cellulose, as the cure takes several days, (not hours like 2 pack).

I used to think that until I hit a bunch of problems and my supplier's rep came out and told me otherwise - he pointed out that the particular clear pre-cat I was using required a second coat within 4 hours or after 24 hours to avoid problems, but cautioned me against applying more than 3 coats as there would be problems of solvent trapping (so an RTFM failure, one might say)...... Some of the stuff I use is acrylic-based and with that there is absolutely no solvent action between coats - I'm told that the amount of solvent "effect" you get is dependant on the catalyst used, but not being a paint chemist I try to get the makers data sheet for everything I spray and stick rigidly to it. I also keep a log of my spraying so that once I've got a technique down pat I can refer back to my log to see how I did it last time. The log contains details of the stain, sealer, colour coat, top coat, finishing schedule (with times) and equipment used, etc. There is a similar thing in Charron's book.

Having thought about it I wonder whether or not the action of sealing the edges has wetted the fibres of the MDF causing them to swell and that the subsequent cracking is where the fibres have shrunk as they dried out?

PS apart from primer or sanding sealer, what do you use on mdf raw edges?

You can use clear French polish as well or if you are spraying a high solids undercoat drywall compound works surprisingly well - partly because it dries in next to no time and sands pretty well. It's also as cheap as chips.

Scrit

 

[ivan]

Thanks again, Scrit. My best guess was that the finish must shrink in curing; if you put it on too thick it will crack, as it's then too strong to stretch. Looks as if this could be it.

I don't think it's due to swelling / shrinking of the fibrous edges, as these are OK on a couple of smaller items that were easier to spray, and so the raw edges didn't get sprayed twice with each coat.

Is Acid Catalised Lacquer (add the catalist before spraying) subject to similar problems?

What's drywall compound? (My experience with plasterboard is limited to screwing it up and watching a clever chap scrim the joints and trowel on a skim coat.)

 

[Scrit]

Is Acid Catalised Lacquer (add the catalist before spraying) subject to similar problems?

I believe so. It seems to have the same sorts of problems you get with 1K stuff, however a 2K can be made more durable as the acid hardener is more reactive and the chemists seem better able to "tune" it. 2Ks frequently have the option of a "winter" hardener which will work at lower temperatures and you can also get "extenders" for them to increase the pot life/open time.

What's drywall compound? (My experience with plasterboard is limited to screwing it up and watching a clever chap scrim the joints and trowel on a skim coat.)

It is a sort of extremely fine plaster and even Wickes sell it (although I've never seen it in B&Q). It came from America along with taper edge plasterboards. When putting-up t/e boards a self-adhesive fibreglass scrim tape is "knifed" into the joint them coated with a thin skim of drywall jointing compound. When dry it is sanded and re skimmed (wider), and then the same process done a third time. Each time the skim is applied wider still and the edges. From that you can see that the compound is lightweight, fine and dries quickly. If you need to do a quick job where there isn't time to do a full plastering job and wait for it to dry it is ideal - plasterboard hung to paint in 24 to 48 hours. They build complete houses this way in the USA and I believe that it's caught-on in Ireland, too.

Scrit

 

[Newbie_Neil]

Hi Scrit

They build complete houses this way in the USA and I believe that it's caught-on in Ireland, too.

Neil picturing plasterboard houses in Ireland and then it rains. :lol: :lol: :lol:

Sorry, I'll get my coat.

Cheers
Neil

 

[Scrit]

Hi Scrit

They build complete houses this way in the USA and I believe that it's caught-on in Ireland, too.

Neil picturing plasterboard houses in Ireland and then it rains. :lol: :lol: :lol:

CORRECTION: They board out complete houses this way.....etc

Happy now? :roll: (hammer) :lol:

Scrit

 

[Sgian Dubh]

Scrit, I have to disagree with you somewhat on coating thickness for pre-cat lacquers. In my experience it's pushing the boat out to apply more than three coats of the stuff at a wet weight of about 10-15 mils thick coming down to 5 mils after drying and sanding. Each coat should dry out to between 4 and 5 mils thick resulting in a potential maximum dry coating thickness of 15 mils or less.

Heavy dry coats over about 13- 15 mils tend to crack across the grain due to the thickness and inflexibility of the polish. The thickness at which cracking (aka known sometimes as chinese writing in bad cases) occurs varies a bit from manufacturer to manufacturer, but in my experience it's safest to get the polish on in two wet coats of about 10- 15 mils thick each.

Of all the nitrocellulose family, including pre-cats and post cats, the only one of that family that I know of that can usually be built up layer upon layer without cracking, crazing or chinese writing problems is nitrocellulose lacquer itself. Slainte.

 

[Scrit]

Scrit, I have to disagree with you somewhat on coating thickness for pre-cat lacquers. In my experience it's pushing the boat out to apply more than three coats of the stuff at a wet weight of about 10-15 mils thick coming down to 5 mils after drying and sanding.

Sorry Sgian, but isn't that what I said? :

.....until I hit a bunch of problems and my supplier's rep came out and told me otherwise - he pointed out that the particular clear pre-cat I was using required a second coat within 4 hours or after 24 hours to avoid problems, but cautioned me against applying more than 3 coats as there would be problems of solvent trapping (so an RTFM failure, one might say)......

Plain fact is that this is not a problem I've experienced for several years because I always stick to 2 or 3 coats in a session..... I know that we all have different experiences in this and that once you've sorted out your gun, booth and thinning levels you note the details and stick to them. I was therefore talking from my personal set-up and I know what works for me. I agree that the coats have to be very thin in order to avoid sags, etc and having solvent trapped in the finish. Interesting what you say about cracking in MDF edges, though as the only time I experienced this in production we tracked it down to a very different reason, but then I wasn't trying to apply a thick coat to seal and finish in one. I have to say, though, the with water-based finishes I'm sometimes still hard pushed to get more than 1 coat on in a session without sags, runs or orange peel.

As to pure nitrocellulose lacquer I thought it was becoming increasingly difficult to source because of its extremely high VOC content and it's impending banning, or have I got that wrong? We'll all be going water based before too long in any case so it's something I'll have to get better and faster at.

Scrit

 

[Sgian Dubh]

You did subsequently Scrit. I missed that later bit and I simply spotted your first response in this thread, viz:-

ivan wrote:
Do you have to be careful how many coats of pre cat you lay down? What about catalised lacquer?

Your reply was "No, not really, but you do have to ensure that you spray second/subsequent coats within the open window and make your coats thin - just a bit more than leaving "orange peel".

The above two quoted paragraphs were what caught my eye and I read where you said, "No, not really," as the only response to ivan's quoted question, "Do you have to be careful how many coats of pre cat you lay down?"

It seems I made the error of not reading all of your responses thoroughly, for which I aplologise. Slainte.

 

[Scrit]

Sorry Sgian

I didn't make it clear enough in my earlier post - I've found years ago that if I tried to lay heavy coats or lots of them then I got the sags.... I've added a not to my earlier post to correct this misimp[ression.

Regards

Scrit

 

[ivan]

I'm finding this very helpful, thankyou all.

(Falsely, as it turns out) secure in my amateur car repair, cullulose background, I thought I'd stick to the familiar, but dip a toe into the world of chemically cureing finishes. My assumption was that anything with a very volatile (quick dry) thinner would be more forgiving of poor spray technique. That right?

If you can build up lots of coats of plain cellulose lacquer (as per guitar bodies for eg.) then it must stay more flexible than pre/post cat, which will crack if the finished film is too thick? That right?

I can see how a 20% solids lacquer applied at 10 mil would dry to 2 mil: I could even check this with a micrometer. Right so far? But how do I measure how thick I'm putting the stuff on wet?

 

[Scrit]

My assumption was that anything with a very volatile (quick dry) thinner would be more forgiving of poor spray technique. That right?

I think that to spray catalysed finishes well you need to get a bit of practice in - so that you can consistently lay down a thin, even coat. There's a knack to it which seems to develop over time (like so many things) and judging how thick a coat goes on is always a bit difficult. I was shown to spray "lean" to begin with so that you are literally just wetting the surface then increase the fluid flow on the gun until you're putting down what looks to me like light drizzle or orange peel - that way there's just enough finish on the surface for the droplets to coalesce and form a contiguous skin. Too much air and you deliver dust, not lacquer, too little material and you don't coat completely. The thinner the skin the faster it will go off (i.e. lose solvent or dry). But because I don't spray on a daily basis I often start a batch off with a couple of test pieces of the same material (scrap) to set the gun up, and I always refer top my spraying log to see how I did the same job last time. This isn't a bad idea anyway as it helps you set the gun up properly before you go near the paying work!

After the test panel(s) are have flashed off if you examine them with the aid of a magnifier it should be possible to determine which has the best material flow-out and overall appearance. The settings from that panel are the ones you need to use for the rest of the batch.

Overall I'd say that catalysed materials are a lot less forgiving of poor spray technique.

If you can build up lots of coats of plain cellulose lacquer (as per guitar bodies for eg.) then it must stay more flexible than pre/post cat, which will crack if the finished film is too thick? That right?

Any chemically hardened finish is probably going to be harder and therefore potentially more brittle than pure solvent finishes. That's the trade off - loss of flexibility for increased hardness.

I can see how a 20% solids lacquer applied at 10 mil would dry to 2 mil: I could even check this with a micrometer. Right so far? But how do I measure how thick I'm putting the stuff on wet?

I don't see how you can, really. Most finish suppliers quote finishing regimes in terms of square metres per litre coverage which will equate to Sgian's figures - if you are well over their usage figures then you're laying it on too thick. For example, take a toybox and work out its surface area - say 2 square metres - then take a material which finishes out at 10 square metres/litre and a spray gun with a pot capacity of 250ml. For each coat of the toybox you will need 2 / 10 litre = 0.2 litres of finish to be used. If you empty the pot your coat is too thick, if there's much more than 50ml left in the pot at the end of a box then either you've missed a bit or your coat's too thin. Simple!

My understanding might be flawed, but I took it that the recoat window was the maximum period you could expect solvent-action recombination of coats to occur within, and that after that if you didn't leave the finish to give-up it's solvent content (i.e. until until the end of the drying time) you would not get such a good bond between the coats as the bond would be a mechanical one onto a smooth surface. Based on what Sgian says, if this bond breaks down, probably as the result of two different coats contracting at different rates (the outer coat quicker as solvent evaporation would be more rapid), you'll get crazing or cracking. I can also see more clearly the need to rub-down between coats where a full drying cycle between them is adhered to - it's partly to remove nibs and partly to provide a mechanical key for the next layer of finish, however we just don't have the time to do that so for us its 2 or 3 thin fast coats in a single session.

From my notes at a course a few years back -

"When someone new to spraying watches an experienced spray finisher they might think 'that looks easy - I can do that'. But it isn't as easy as it looks. Sprayers are very skilled because it requires a great degree of skill to be a consistent spray finisher with modern thin-coat materials.

- the line of sight down the sprayer's arm past the gun and down to the spray surface makes it difficult for the sprayer to judge gun to part distance. Consistent distance is imperative to achieve consistent results.

- a high degree of motor skills are required

- good vision is required

- spray techniques are not natural to human anatomy and are contrary to natural body movements

- spraying requires concentration and constant mental awareness"

Go and watch a full-time sprayer in a furniture spray plant and you'll see what I mean. Those guys motor. On that basis I don't rank too high, but I get by.

I suspect from what you are saying that you may be holding the gun too close to the work and so laying down the material too rapidly and too wet. Your gun to work position should be around 6 to 12in from the part for many guns or that your traverse speed is too slow.

Sorry to make this a long post but it's all been rattling around in the old noddle today. Sgian's succeeded in getting me to think about something I though I was doing reasonably well at - and on which I obviously need to improve.

Scrit

 

[Sgian Dubh]

I forgot to answer your earlier question Scrit. I'm not aware that nitrocellulose lacquer is soon to be made unavailable. It may be the case, but I haven't heard that. I'm resisiting the general move to water based products for as long as I can on the basis that so far they mostly don't look right as they all tend to have one or more of the faults of looking cold, milky, dead, flat, plasticky, or blue tinged.

I spent time looking at Ercol's system down in Princes Risborough a year or two back and and got to know their spray shop manager somewhat-- he is a rugby player too, and we even played in the same team once or twice. They get their water based finishes made specially by a large manufacturer. It doesn't look particularly good to me and my acquaintance admitted the system is problematical and difficult compared to the previous mostly petroleum based polishes they used to use. This even though they worked directly with their supplier.

"Sgian's succeeded in getting me to think about something I though I was doing reasonably well at - and on which I obviously need to improve."

That wasn't my intention. As far as I can tell without ever having met you you seem more than knowledgeable enough and have techniques that work.

There are paint or polish gauges available incidentally. I haven't looked for one for years but they used to give them away with batches of polish I picked up when I lived in the US. I haven't seen one given away or available at polish houses since I moved back to the UK, but I haven't looked either, and I don't even know if I've got any of the free ones I was given anymore.

Basically they are an aluminium plate about the size of a credit card. The straight edges are notched, and reading from left to right starting in one corner and going around each edge and back to the first corner the first notch is shallow at say 2 mils with graduating depth of notch going all the way up to perhaps 20 or 25 mils. Spray on a coat of polish and stick the edge of the polish thickness gauge into the wet film to see which notch the applied film surface lines up with.

Best to do this test (with fast drying finishes anyway) on sample pieces or on obscured finished parts. Don't ask me how I know that, ha, ha-- ha, ha, ha. Slainte.

 

[ivan]

Thanks again for your posts which make very interesting reading.

Interestingly, I mentioned their water based finish to the mfr., bearing in mind the requirement to spray a 4m run of built in wardrobes with only fan assisted cross ventilation from the next room. (The fan was in the next room pressurising the house, the windows next to the built in open and covered with glass fibre paint stop) His technical man said "you won't like it". After telling him of my past limited experience, he suggested the pre cat.

Some overconfidence on my part was due to conveniently forgetting that car bits are generally simple surfaces, unlike furniture with limited clearance and all inside corners. I found these difficult even though the finish was now in a pot on the floor. Being built in I couldn't spray with the backs off and was peering through some overspray when coating inside corners. I found it hard to blend corners into flat panels at arms length and almost blind; it was here in these dodgy areas I got some cracking (and less than perfect finish...) fortunately it doesn't show. I certainly need practice!

More obvious is the cracking on the cabinet edges, which did get 3 coats twice. There was no problem with the doors, which were sprayed in front of the window, flat, much as if in a booth. (edges 3 coats once)

My testing was limited to gun adjustments whist spraying onto paper; next time there'll be some test pieces too.

Thanks again

 

[mrbmcg]

Can I just take the time to thank the contributors to this thread. It's the most informative piece of info on spraying precat that I have read.

Thanks guys