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I’m testing to see how well shellac works as a grain filler on Mahogany.

I have brushed on several coats of 2# shellac using a foam brush. I have sanded between coats but I’m still getting those pins size shiny spots.
At this point, I’ve sanded nearly threw the finish back to the wood, and those pin size divots are still there.
Are these tiny low spots caused by air bubbles created with the foam brush I am using? I will spray whatever finish I end up with but I don't want to go to the trouble on a test piece.

Thanks

When using shellac to fill pores, it takes several coats. But depending on the finish you are using, you may not need the pores completely filled, as long as they are completely sealed. The first two coats of finish should fill them completely.

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A man hears what he wants to hear, and disreguards the rest. Paul Simon

There has been 7 brushed on coats up to this point. I was going to try a shellac french polish, so I am assuming starting with a flat surface would be important

Thanks

I think Robbie O'brian has a u-tube on pore filling with shellac.....basically, using 320 grit to develop a dust, then apply shellac, rubbing into pores....do only small areas at a time. I use it and it works.

If you are planning to French polish and want the pores to be filled there is an extra step in the process. Forgive the question, have you done FP before or is this the first time? If it is the first time, are you following any particular tutorial? Typically you would use pumice and alcohol to do the pore filling with the FP process. Or. . . use any of a number of other pore filling techniques before you start FP (it may be too late for some of those with that much shellac on the surface). Of course there is nothing wrong with not filling the pores if that look pleases you.

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Bryan Bear PMoMC

Take care of your feet, and your feet will take care of you.

Yes, this is my first attempt at French Polish and I have watched a bunch of youtube tutorials.

In my attempt to flatten it the shellac has been sanded almost back to the wood. I assumed that if I could flatten the surface with just shellac I could skip the pumice. I still don't understand why I can't get it flat. What's causing the pin size divots? I have flattened shellac before, I don't understand why it's not working. Maybe I was not looking as close as I am now

You can get there with just shellac but it will take longer. Have you flattened shellac on open grained woods before? It sounds like you are describing pores that are not yet filled. If you sand back too hard, you could even expose new pores. Also remember that shellac will shrink a little bit while it is drying. Sometimes you can sand back to level and then see pores when you come back later.

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Bryan Bear PMoMC

Take care of your feet, and your feet will take care of you.

No these are not wood pores. They are pin size low spots the in the shellac above the wood.

Hmm, pictures? I know it is probably really difficult to photograph. . .

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Bryan Bear PMoMC

Take care of your feet, and your feet will take care of you.

That's a completely different problem. I think a photo mat be necessary for good advice, my first guess was way off.
Yes you need a flat surface for FP and 7 brushed coats should have been enough to fill the pores.

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A man hears what he wants to hear, and disreguards the rest. Paul Simon

Could be that the shellac is sinking back into the pores at those spots. The limitation of shellac for what you are doing is that it has very little mass or space filling capacity. Usually people do this type of pore fill by using shellac (or egg white, or danish oil, or whatever) to wet sand the bare wood. This raises fine dust from the wood and forms a slurry which you work into the wood. The pores are filled mainly by the dust, and the shellac or other compound is just working as a binder to keep it there as it dries. In your approach you are requiring the shellac to do all the filling and it is just not a very good at doing that. But it should eventually build enough mass to fill voids and form a level surface.

I also wonder if your foam brush is allowing a small air bubble to be trapped in those depressed spots and the shellac bridges over the top of them. You could try a different application method.

Yes, I use fine pumice with thin shellac and FP and get a really good fill.

I think I duplicated your issue. As a career painter, and new ukulele maker I was curious. I was working on a test piece of scrap Honduran Mahogany from the back plate, applying multiple coats of wax free shellac sanding sealer for pore filling. It went fine until the 4th coat and then as the coating was drying tiny bubbles appeared all over the surface. That would be caused by gasses escaping from inside the wood, most likely water vapor displaced by the alcohol solvent and the coating was thick enough not to be able to let it pass through, so it was captured as tiny bubbles about the size of a pin head or smaller. If you sanded down to bare wood, that couldn't happen though, and could be the result of using a foam brush instead of a bristle brush.