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Hi, thanks for looking at this post!

I mainly do repairs but last year decided to give a guitar build a go and of course dropped myself in at the deep end with fancy 'hippy-sandwiches', figured woods and unconventional shapes. It all went pretty well until it came to finishing, I started by applying a nitro sanding sealer. Did 4-5 coats sanded it down and then started with the nitro clear but it soon became apparent that the grain was not filled properly .


Help please ?



https://www.youtube.com/watch?v=PNzUTglqV_g

Nitro sanding sealer and topcoats are basically solids suspended in a solvent based carrier. When you spray this solvent has to be released for the material to dry. As it dries the film shrinks and thins. In a pore, the material will settle and a seemingly flat surface can show pore depressions much later. Complete evacuation of the solvent in a pore takes a VERY long time.

You can use sanding sealer to fill pores BUT...you have to wait several weeks after spraying before leveling and topcoating.

And there's another issue. The cool nature of nitro is that it eats into itself when a fresh coat goes over a dry coat. When using sanding sealer to fill pores you think you have a perfectly smooth surface on which to spray the topcoat...and it IS smooth...but spraying wet topcoats onto that surface eats into the cured sanding sealer since they are both nitro based...and it eats deeper into the filled pores than the area surrounding the pore. Essentially, when spraying the topcoats you end up re-introducing solvent into the upper strata of the filled pores...even though you waited all that time for the sanding sealer to dry.

Eventually...I learned to spray the sanding sealer, let it set three months, level it, spray the topcoats, let it set three months, and then do the final leveling and polishing. The result is very nice...but it's a pretty stupid process if you ever want to get anything done.

The best way to use nitro topcoats is over an epoxy pore fill or some other kind of inert material. Epoxy is a two part system that cures rather than dries....meaning two chemicals come together to harden into a solid. It does not contain any solvent and therefore will not shrink once it is cured. Although the solvent in subsequent nitro topcoats still has to blow off...it does so evenly.

You can fix this and start over if it's just a clear coat. Simply cut a BUNCH of paper towels into quarter squares and scrub the stuff off with acetone. This makes pretty short work of getting the entire finish off. If you have a tint in the finish, however, you are kind of screwed. Acetone can leech the color down deep, depending on the wood.

_________________
I read Emerson on the can. A foolish consistency is the hobgoblin of little minds...true...but a consistent reading of Emerson has its uses nevertheless.

StuMusic

Wow, thanks for all the information that's exactly what I wanted/needed to know. Top bloke.

Anytime.

I thought I'd add that if you DO scrub off the finish with acetone...make sure once you think it's clean to go over it several more time with clean towels and acetone. You want to get as much residual stuff out of those pores as possible. Epoxy is very aggressive on a surface and will stick to almost anything...but the cleaner the better.

_________________
I read Emerson on the can. A foolish consistency is the hobgoblin of little minds...true...but a consistent reading of Emerson has its uses nevertheless.

StuMusic