Sealers and pore fillers

[GregShelton]

Hi everyone,

I was wondering if different finishes require the use of a specific sealer? For example, if I am using shellac as my finish, must I only use a shellac sanding sealer or if I am applying a water-based PU finish, must I look for a PU sealer?
Are there combinations that are preferable or those that are best avoided?

Thank you all again for your great help!

 

[ED65]

In normal circumstances no finishes require the use of sealer . The first coat of any clear finish can act as the 'sealer' (although none of these genuinely seal the wood, hence the quotes). Shellac and cellulose (lacquer) sanding sealers are basically just dilute forms of the two finishes, with additives, so if shellac or lacquer will be the final finish then they're almost redundant.

But they do have uses when intending to use something else as the main finish. One reason you may choose to use shellac under water-bourne poly is that the latter is generally "water white" so it colours the wood almost not at all, which in some cases leaves it looking a little washed out, pale/wan. A coat or two of shellac beforehand colours the wood slightly, often stated as "warms it up", which is often thought beneficial although of course it depends on the look you're after.

If you do want to use shellac for this purpose I should add that sanding sealer is not the best way of buying it, so if you haven't purchased any yet then I wouldn't. It's best to buy shellac and make it yourself by dissolving it in methylated spirits but if you don't fancy that then look into SealCoat by Zinsser.

 

[Cordy]

ED 65
Do you mean This ?

If I buy Shellac flakes to mix my own with meths; what type ? what ratio ?
.... to use on Redwood, Euro Oak and Walnut

Cheers -- John

 

[custard]

Sanding sealer has a number of benefits. It contains additives that make sanding much easier, in particular you won't get little lumps of the finish clumping up and scratching the surface. Shellac sanding sealer provides a barrier against oils or resins in the timber migrating out and ruining your top coat. It often provides a little bit of grain filling, for some timbers this might be all you need. In addition it provides a much more uniform surface for subsequent finishes, most timbers will absorb finishes more readily in some areas than others, which can result in patchy or uneven finishing.

Do you need a sanding sealer for your specific project? Depends on the timbers your using and how it'll be used.

 

[custard]

ED 65
Do you mean This ?

If I buy Shellac flakes to mix my own with meths; what type ? what ratio ?
.... to use on Redwood, Euro Oak and Walnut

Cheers -- John

I've only used Zinsser Seal Coat a couple of times, I didn't like it because I found the cap quickly gummed up and became almost impossible to remove, especially as the bottle was ultra thin plastic which felt like it was in danger of splitting from the force required to unscrew the cap. If you do use I'd suggest decanting into screw top glass jars and then use it up as quickly as possible.

I don't know off-hand what cut Seal Coat is, but it was clearly very heavy, I'd guess something like a six or seven pound cut.

 

[GregShelton]

Thanks for the replies.
I'm not sure if the question was addressed to me Custard but in terms of my project, I'm not working on a specific project, I'm just trying my hand at finishing for the first time on different pieces of ply that I have veneered with a range of different wood types to see what happens when I apply a range of techniques and finishing products. I had the idea that I may need a sealer as I have found that oak in particular is not filling up despite lots of careful sanding and a good few (thin) coats of PU finish.

 

[Sgian Dubh]

I had the idea that I may need a sealer as I have found that oak in particular is not filling up despite lots of careful sanding and a good few (thin) coats of PU finish.

Do you mean that the appearance of the wood's surface is still heavily textured, oak being a coarse textured wood species?

If that's the case then I generally don't advise aiming to fill the open grain pores with the finish, although it can be done, but doing so can result sometimes in less than desirable characteristics. In that situation, and assuming you're looking for a smooth surface, i.e., with the pores filled giving a smooth texture to the touch, I suggest you consider a grain filler which can be applied either before any finish coats go on, or after an initial coat applied specifically to seal the wood.

Here's a link to one type of grain filler (Rustins), although there are plenty of other manufacturers of similar products. The link I've provided indicates this supplier seems to be out of stock, but there are plenty of other suppliers out there: https://www.restexpress.co.uk/acatalog/ ... iller.html
Slainte.

 

[ED65]

ED 65
Do you mean This ?

Yes. That's pretty expensive by the way, I think I can get it slightly cheaper here paying retail! This is really saying something as paints and finishes tend to be very expensive here.

Just did a quick search, currently it's slightly more on Amazon UK but decoratingwarehouse.co.uk have it for £20.95 for a litre so you might be able to save a few bob if you decide to try the SealCoat.

If I buy Shellac flakes to mix my own with meths; what type ? what ratio ?
.... to use on Redwood, Euro Oak and Walnut

Depends on what you want in both cases.

Colour or grade wise you pick what you want from the wide range of colours that shellac comes in, which to an extent depends on what you want the shellac for, a standalone finish (either for French polishing or for brush or spray application), as a sealer, e.g. before using grain filler, or to seal off silicone contamination, or as a bonding coat, e.g. between two dissimilar finishes.

As to mixing ratio there are some standard 'cuts' of shellac, with a 3lb cut being considered a useful middle-of-the-road mix although some consider this a tad on the heavy side and will commonly dilute it. SealCoat is a 3lb cut (or was at least, I believe they no longer specify).

But really you can use shellac as thin or as thick as you're comfortable in applying it or as suits the purpose. At the two extremes there's a "spit coat", which can be very dilute indeed, to a mix more concentrated than a 4lb cut.

The very thin spit coat would generally be used as a quick sealer or surface hardener, allowing fuzzy grain to be more easily sanded off (one of the main purposes of commercial sanding sealers). A heavier cut will allow a faster build if applying by brush.

Shellac can work well and look great on all three of those woods you list. As you might already know the walnut especially can be made to look amazing using shellac, although you will want to oil the wood first if you'd like to maximise any figure.

 

[ED65]

...I'm just trying my hand at finishing for the first time on different pieces of ply that I have veneered with a range of different wood types to see what happens when I apply a range of techniques and finishing products.

Something to note here is that veneers sometimes don't work quite the same as solid wood of the same species because of the way the veneers are formed.

I had the idea that I may need a sealer as I have found that oak in particular is not filling up despite lots of careful sanding and a good few (thin) coats of PU finish.

I presume you're reading finishing sources and this should be made plain but in case you haven't read this yet, sanding alone will never fill up an open-grained wood. Only if you bind the sanding dust and have it build up in the recesses, as some do when oiling (oil is not the best thing for this purpose BTW), will you be able to get the grain to fill in a reasonable time.

You can grain fill using just the clear finish but you use – or waste, depending on how you want to look at it! – lots of the finish doing so. IMO this is something best left to luthiers and makers of high-end boxes and furniture; there are numerous more user-friendly ways of filling grain and they're all much faster and the wood can still look stonking.

One of the oldest techniques going for grain filling, using plaster of Paris (either plain or slightly tinted), has largely gone out of favour today but it is still worth considering. It's cheap, simple and easy, versatile and lacks absolutely nothing in the looks department:

 

[custard]

One of the oldest techniques going for grain filling, using plaster of Paris (either plain or slightly tinted), has largely gone out of favour today but it is still worth considering. It's cheap, simple and easy, versatile and lacks absolutely nothing in the looks department:

Grain filling with Plaster of Paris is cheap and fast, but the risk is that over many years the oil migrates out of the plaster into the wood, leaving white specs trapped in the pores below the shellac surface. I see you pulled those photos from a restorer's web site,

http://pianomaker.co.uk/technical/filling/

Maybe a restorer or someone in the repro trade could justify it on the grounds that it was the original method, but personally I prefer to use a different approach.

 

[GregShelton]

Only if you bind the sanding dust and have it build up in the recesses, as some do when oiling (oil is not the best thing for this purpose BTW), will you be able to get the grain to fill in a reasonable time.
You can grain fill using just the clear finish but you use – or waste, depending on how you want to look at it! – lots of the finish doing so. IMO this is something best left to luthiers and makers of high-end boxes and furniture.

Hi ED65,
In regards to your reply, I would like to ask if it is possible to fill grain by mixing the finish you intend to use with sawdust (and a little of the appropriate diluting fluid) to create a filling paste? I have to say that I have already made some up and applied it just to see but I haven't sanded it back yet!
Also, I am indeed interesting in fine box making so I am open to time-consuming and painstaking methods of application!

 

[Phil Pascoe]

As it's basically end grain it will show darker than the surrounding wood - some open grain multi coloured woods it's ok on but it'll show on others.

 

[ED65]

Hi ED65,
In regards to your reply, I would like to ask if it is possible to fill grain by mixing the finish you intend to use with sawdust (and a little of the appropriate diluting fluid) to create a filling paste?

I presume by sawdust you mean sanding dust, in which case yes dust + finish is exactly what some use. Generally you'll want the dust to be very fine, generated by 240 grit or above.

This works best with varnish but can be done with a drying oil. The reason oil isn't the best thing for this is that it dries slowly and not very hard; bound with varnish, or a mixture like Danish oil that contains varnish, the filler dries faster and harder. Regardless of what you use the paste tends not to store well so will likely need to be mixed fresh each time it's needed.

BTW to save on sanding time you can scrape the majority of the excess off the surface while still workable using a scraper with corners rounded for safety. An old credit card can work if you don't have a suitable metal scraper to hand.

Somewhat the same result is achieved by sanding in the wet finish, the filler is created on the surface and naturally packs into the grain.

Also, I am indeed interesting in fine box making so I am open to time-consuming and painstaking methods of application!

French polishing will be right up your alley then

 

[custard]

Top quality, traditional french polishing filled the grain by grinding the surface with pumice. It creates a really tough, stable and enduring amalgam of pumice, shellac, and finely powdered wood that is forced deep into the pores. If you've got the time and the patience you should try it, the materials cost almost nothing and it yields an especially bright and sparkling result. The downsides are it's a long job and on marquetry it can transfer the colour of a darker wood into the pores of adjoining paler woods.

 

[GregShelton]

Thanks for the great replies everyone.
In terms of the pumice technique, could I please ask exactly how it is done Custard?
Thank you all!

 

[custard]

Get a pounce bag (a well washed good quality cotton hanky) and some 4F pumice. Apply a couple of heavy coats of shellac. Flick the pounce bag from 12-18" above the workpiece to release a fine, light shower of pumice on your polished surface. Work with thinner shellac (or even neat meths) and you'll hear/feel a slight scratching as the pumice grinds away the surface of the wood and your polishing action melds it into the shellac and packs it down into the pores. Too loud, like coarse dry grinding, means you've applied too much pumice, silence and smoothness means it's all used up or you applied too little. Use the bare minimum of oil or none at all, before applying oil try recharging your rubber or adjusting the speed and pressure of your strokes. Excess oil is the easy way out, experienced polishers do use oil, but far less than you'd think.

I'm not a fan of the formulaic approach to french polishing. I learned in workshops where polish was in daily use. The workshop would start by half filling a large jar with shellac flakes and then just covered them with alcohol. The individual polisher would decant some of this and then cut it further according to personal taste and to the specific situation and conditions they were facing. Nobody ever weighed anything. It's a crude approximation of that reality that has filtered down as "2lb cuts", "1lb cuts", and what have you. Okay, you have to start somewhere, but if you learn to develop a feel for tinkering with blends then you'll be better off in the long run.

By the way, pumice wears through the coverings on your rubber in pretty short order, some polishers put two or three outer wraps on and then adjust and finally discard them as they wear out. Treat with a pinch of salt all the voodoo stuff about "ten figure of eight circuits followed by six circuits of tight circles", I've never seen two polishers who use the same method and yet they all seem to produce pretty good work. Just follow the old advice, "take care of the edges and the centre will look after itself".

You'll know when you're done, hold the workpiece up to glancing light and there'll be no more pores left to fill. Leave a couple of days to harden and then check again.

There are plenty of YouTube videos out there to give you more of an idea. Just remember it's the internet so it's full of guys who are long on opinions but short on experience. Common sense really, but try and find videos from people who show ample evidence of successful projects that are as closely aligned as possible to your own plans.

Good luck!