Shellac adhesion

[Dave M]

We keep getting told that pretty much any finish will stick to shellac but what about the other way round?

That might sound a bit silly but I am in the situation where I have scraped off a nitro finish because I had a couple of little knot holes that I thought were filled but weren't. I had tried filing these with CA on top of the nitro but spraying nitro over this wasn't behaving. So after sanding clean I put on a couple of coats of shellac in order to seal any ca left around the holes. However spraying nitro over the top is still showing the original circle where the ca was.
So is the shellac failing to cover either the remaining trace of CA or a trace of nitro? Or is something else entirely going on?

Puzzled of Somerset

[kiwigeo]

Dave, there are some who use CA as a pore fill and then out a coat of shellac over same so I wouldn't think that shellac is incompatible with CA.

Shellac over nitro......I believe it can be done but is advised against as you're putting a soft finish over a hard finish and things like crazing can become an issue.

What sort of surface preparation and pore filling was done prior to the original nitro finish?

[Trevor Gore]

Something else entirely is going on, basically to do with shrinkage rates and solvent (re-)absorption.

CA can work as a drop filler on bare wood, but it needs to be fully cured before other finishes are applied over. Even then, the next finish does not absorb into the surface in the same way over the CA as it does over the wood, which makes this techniques only useful for things like very minor purfling gaps where the CA is fully scraped off the surrounding wood. In inter-coat drop fill type situations you will nearly always see some sort of "bubble" where the CA was, which then turns into a depression if you sand it too soon. Drop fills with lacquer can react in much the same way. Moral of the story: don't use CA for drop fills in FP or nitro. If you drop fill nitro on nitro, you have to drop fill on very new nitro so the shrinkage of the fill is similar to the shrinkage of the base coats and even then you will likely need more nitro sprayed over the lot and more sanding sessions to make the drop fill disappear for ever. A drop fill on nitro, leveled after 3months drying of the drop fill, will show 3 months later (if you look hard).

[Dave M]

Martin, Trevor thanks for the useful insights - as ever.

I found I had been daft. When I thought I had cleaned back to bare wood I was actually nowhere near as I found out today when I went to remove it all again - crikey I'd put a lot of lacquer on that back! Anyway I had a mishmash of nitro, CA shellac and more nitro so no wonder it wasn't happy. Anyway today I got it really cleaned back, couple of shellac coats and the first spray of nitro looks OK.

For interest this is what it looked like:

and the knots are only small:

I thought they would add interest as little features - next time I shall refuse the timber!

Cheers