Old School Worktop Patchy Finish

[Mark S]

Hi, first post here apologies if asking for help is poor etiquette or this is the wrong sub-forum.

I've just started my first 'proper' wood work project - building a desk for a home office. Nothing fancy just a table top and square metal tubing in a rectangle shape for support.

I managed to find an old used school worktop at a second hand/recycling place in Devon that is perfect. I was told it was African Mahogany and my father-in-law reckons it might be Sapele - I am not sure at all. It is a dark hardwood with a good weight, colour and grain made out of two pieces joined together.

The piece was a good 2.8m long by 600mm and about roughly 3cm (inch) thick. I've cut it down to 1.8m for the desk with the spare metre for maybe some shelves to match (haven't decided yet).

As it was an old school worktop it has lots of sign of abuse and use - some of which gives it the character I like. However it has some glue residue along one edge and paint near the edge on the bottom. This picture is of the top before I've done anything apart from cut the piece down (note the glue edge):

End grain:

Close up of grain:

I decided best course of action would be to sand down the entire piece to remove the glue and residue from the edge and also remove all the little nicks, knocks and try to smooth the piece and remove anything that might have soaked into the wood - I don't think it ever had a varnish or protective coat on it. I work with a random orbit sander first as hand sanding the glue was taking an age. I then finished with increasing finer grit up to 120 and then 240 (got carried away and love the feel of the wood).

I finished the edges and rounded the corners. I cleaned the surface with a hoover and brush, then used a tack cloth to get rid of any dust. I used white spirit to clean and get an idea of what the wood would like when varnished.

I didn't want to change the colour too much, really just wanted to protect the wood from water, drink spills that sort of thing so thought a clear satin varnish would be OK. (Ideally I want a protective coat that didn't change the colour as much as the varnish did).
I applied a coat (probably a little thick if I'm honest) and the piece darken (as expected) but not uniformly, more on that in a bit.
Below is a picture comparing the varnished coat after drying with a raw wood from the off cut:

You can see from that picture some patchy-ness of the varnish on the wood. Below are better pictures of my problem:

As you can see the varnish isn't consistent, I've tried googling but my inexperience isn't helping. It's not the same along the grain, so one part soaks up and then as you can see the next bit hasn't and is much lighter. I actually quite like the dark look but the light patches are ruining the finish for me.

Maybe I didn't sand enough? It felt the same smoothness all over and I couldn't see anything wrong before.
Maybe the white spirit haven't all evaporated? It looked dry, maybe the white spirit stopped the wood absorbing in certain places.
Maybe oil or grease left in the wood? The wood was even coloured after sanding without any obvious differences that I can now see with the varnish.

So I've decided that I don't like the coat and so have started sanding it back to raw wood:

It is coming off OK so I should be able to get back to the same start point.

What I am looking for is advice on why this happened, ways to avoid it once I'm ready to go again and any other advice.

(and if anyone knows what wood that would be great too!)

Sorry for the long post for a first timer, thought lots of pictures and detail would make it somewhat interesting!

Thanks,
Mark

 

[John15]

Don't worry Mark. Someone experienced in finishing will be along to advise you.

John

 

[Mike Jordan]

African Mahogany was a name used for Iroko some years back, the end grain shot looks very much like Iroko to me. Any problems finishing this are usually related to its oily nature.
The only problem I have had is the slow drying of varnish, this is a result of the oil content of the wood. Cellulose sanding sealer as the first coat allows you to see any colour problems in about 15 minutes but needs to be treated with care because of its flammability.

 

[Mark S]

OK, thanks for the identification.

Is the oily nature of the wood the potential cause for the discolouration? The oil on the wood preventing even abortion of the varnish?

Would the cellulose sanding sealer help even out the varnish stain and colour?

 

[Mike Jordan]

Since the timber is old it may have been subjected to a great deal of exposure to sunlight which may affect the colour and absorbency of the surface. Varnish tends to sit on the surface of Iroko rather than being absorbed very far. You are perhaps doing the best thing by sanding back the surface for another try. Cellulose acts as a paint stripper if you put it over oil bound paints or varnishes but it's OK to coat with cellulose sealer first and varnish over with oil bound after its dried.
One advantage of cellulose is that it dries very rapidly so that you can see the effect on the timber surface. It's definitely an outdoor job though unless you are into solvent abuse!

 

[Mark S]

OK.

Would another type of finish be more appropriate for Iroko? I'm going to be using the desk in my home office which will sit my computer and printer. I like the colour and look of the wood without any varnish but want to protect it from use.

I've read about pre-stain wood conditioners that might help with getting a consistent finish. Is this something that could work?

 

[Mike Jordan]

I have no experience of pre stain conditioners so can't comment on that. Is the varnish you are using oil based or one of the water based types. My experience of water based varnishes has not been totally happy. It seems to be much more difficult to use on large items like table tops, over brushing seems to spoil the finish and leave marks which remain in the finished surface. I find that it's vital to move really quickly and resist the temptation to brush it out like an oil bound finish.

 

[Mark S]

It's a wickes clear satin poly varnish. It says it's solvent based

 

[Phil Pascoe]

African Mahogany was a name used for Iroko some years back ...

Really? Must have been before the 50+ years I can remember.

 

[Mike Jordan]

Hi Phil
I think you may be right, I've heard it called African Teak and Poor Mans Teak, it's also known by a variety of what I assume are African names. Milicia excelsa is its posh name l think.
I first heard it called African Mahogany about five years ago when one of my boating customers told me that his grandfather had left him a shed full of it that he wished to sell. On looking at a sample he provided it proved to be Iroko.
I've since seen a couple of similar references to the name.
Let's face it timber merchants tend to be a bit inventive with names, an example being boring Poplar, I understood that it was only good for firewood or making clogs but as Tulip or canary wood some people sing its praises as nice timber.

 

[Phil Pascoe]

Afaik it's Khaya sub species that are usually referred to as African Mahogany, but many moons ago I've known carpenters use the term as a blanket cover for them lumped in with Sapele and Utile.

 

[Mark S]

Anyone have any other ideas for protecting Iroko other than a poly varnish? Ideally without staining/changing the colour.

Could wax work?

 

[Mike Jordan]

You did say you were happy with the look of the untreated wood so perhaps a wax finish or Danish oil would suit you. It would be handy if you could try the effect on the underside before taking the plunge.

 

[profchris]

Looking at those pictures I suspect you haven't sanded through all the old varnish. I get this when recycling wood.

A quick test is to splash some white spirit on the sanded surface - any remaining old varnish will show up clearly.

To get the colour even, I'd start with 2 or 3 wiped on coats of shellac (shellac sanding sealer or ready mixed French Polish). Wiping on prevents the end grain absorbing too much. Apply 2 or 3 hours apart, lightly send after each as the grain will raise a bit. Dark French Polish can be used to deepen the colour, though it looks good as is. Varnish will sit well on top.

 

[MARK.B.]

Just a thought,perhaps as you have a sizable off cut you could trial your finish on that until you are satisfied then repeat with the larger piece.

 

[Mark S]

I'm not sure it had any old varnish. When I started sanding the wood first time round the colour and character didn't change as I sanded it smooth.

After looking into Danish Oil I think this might be more suitable. I like the colour and look of the raw wood but just want to protect it. But I'll see when I've finished taking off the coat.

 

[Mark S]

Just a thought,perhaps as you have a sizable off cut you could trial your finish on that until you are satisfied then repeat with the larger piece.

Yeah I probably will do that, I plan to use it but I the desk is why I got the wood so better to test on that suppose.

 

[MARK.B.]

No reason why you cant still use the off cut for shelves etc,it just needs sanding back and it has got to be easier and quicker to
practice on the smaller piece.

 

[kevinlightfoot]

Iroko was commonly used in school science labs,you probably have one of these tops,they were subjected to a lot of abuse including chemical spillage which may be causing some of your problems,perhaps a wax finish may be your best option but better men than me may have a solution for your problem.

 

[Jamster21]

Unlikely that I am a better man than Kevin but I have done a similar thing with very similar looking timber and I just waxed it a couple of times - one thing though, I noted that it remained quite "pore-y" even when sanded down through the grits so gave it a wipe with white spirit after the first coat. Comes out well - thinking was that as a chemistry bench it'll have had some stick chemically and / or thermally and any differential absorbtion of the wax is easy to touch up with another coat...