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Like a dummy I used a traditional painters brush when applying shellac to a mdf surface top. I probably did five or six coats at this point I’ve lost count. I can now see obvious stroke marks from the brush. I just realized if I had used a foam brush those stroke marks may not be as visible.

Is it even possible to hide those marks if I apply a few more layers of shellac using a foam brush or is it not worth it? I guess at the end of the project I can live with them being present, I’m more curious for future projects.

I do plan on sanding the final coat lightly using a 600 grit or higher if I can find it, so ultimately not sure how much that would knock off. But from previous layers I’ve applied and then sanded using 400 grit those stroke marks are still visible.

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Dry shellac remains soluble in its solvent. What this means is you can use the solvent to redissolve and redistribute the surface of a shellac film. Although it's a gross simplification this is the basis of French polishing, where the application tool (the rubber) constantly moves around and partially redistributes the shellac right at the surface, and eventually polishes that surface.

So in theory at least you could switch application tool to a rubber and start working the surface with it and eventually it would flatten out, partially from taking off the high spots but also from simultaneously building shellac in the low spots.

But since you've applied quite a film of shellac already there's a much simpler, and quicker, alternative: you can simply sand it flat (or at least flatter, it doesn't need to be perfect to proceed to a subsequent step).

I would have advised starting just a little coarser, but since you already have some 400 grit start there. Back the paper1.

Sand carefully and try your hardest not to let the sanding block overrun edges; take particular care at the corners as these are easily sanded through (if this happens don't panic, with shellac you can do touchups really easily and seamlessly). Eventually you should get down through all the brush marks to a level surface. You'll be able to tell when you've sanded enough as the surface will be uniformly matt, with zero shiny spots.

After this you have options. You could try brushing on a final layer of shellac, but it's actually really difficult to brush-apply more than maybe 2-3 thin coats of shellac and get a really good surface even with the right brush2. Brush application gets progressively harder the more shellac is on the surface.

French polishing
So instead, you could now switch to a rudimentary rubber3 and apply the last couple of coats of shellac that way, using as much oil as necessary to lubricate the rubber. See French Polishing MDF (short version) I wish the long version were still up but this will give you an idea of what you can achieve without too much time and effort, and without tons of practice. Don't be intimidated, trust me it is possible for a first-timer to get a really good result :-)

There are many more guides to French polishing online to get a little more detail and to see more of it in action. If you find you enjoy the process you can dive deeper by hitting the books. But I caution that at heart it is really a pretty simple process that is made to seem much harder than it is in some written sources; one of the secrets being to use plenty of oil. Many traditional guides suggest the absolute minimum is 'correct', and invariably then go on to state that excess oil is hard to remove which couldn't be further from the truth.

Perfect the existing surface
Alternatively, and assuming you didn't inadvertently sand through in any areas that need touching up, you can buff up the existing surface. If you need a pretty good gloss you can eventually polish to a high shine using a succession of finer grits. See these previous Questions for some more detail:
Leveling a finish/finishing the finish
How do I achieve a "piano black" high gloss finish on wood?


1 Because this is a flattening operation and not just scuff-sanding you need to back the paper with a firm surface of some kind, and not just your fingers. If you don't have a commercial sanding block or other kind of sandpaper holder just a small piece of wood will be work fine, a piece of 2x4 that fits into your hand neatly is perfectly sufficient. If you only have MDF on hand a small piece of that will also work fine, but ideally I'd laminate two pieces together to make it thicker and easier to hold.

2 The ideal brush type for this would be a fine-bristled synthetic brush, perhaps 2" wide. Both nylon ("Krylon", typically orange) and polyester (usually white) brushes are suitable. A foam brush is very much not ideal for a built-up shellac film; they're actually quite crude applicators and can leave substantial 'brushmarks' despite their fine texture, so are best suited to finishes that dry more slowly which leaves time for the finish/paint to level out on its own.

3 Just a square of clean, well-washed cotton or linen carefully pulled taut over the reservoir — traditionally a wad of cotton wool (although in the trade it was never called that!) but a scrunched-up tissue or paper towel work just as well LOL Any decent guide will give fuller instructions on how to fold the cloth to make a tight, firm rubber (either the more traditional teardrop shape or a simpler round one).

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