I have never tried it so I don't know, but...

Even if others have had success doing poly over lacquer, your success will depend on using the same materials and methods. There is a very wide difference within the ranks of polyester and lacquer finishes.

I would try a test first. I would do a sample piece, doing it the same as the existing guitar finish. Then I would divide the sample into sections and try different materials/methods until I got the results I wanted.

Good luck!

James, I have never tried to top lacquer with polyester but I would think it would not work well. Like you, I have topped cured polyester with lacquer and urethane but not the other way around. Maybe if you were able to seal the lacquer with an isolante, or barrier coater or polyester sealer like you usually use to seal exotic woods, the polyester would lay on there. As usual, an actual spray test is going to be your best friend.

Alternatively, if I decide that spraying the polyester sealer is too risky, I am considering spraying vinyl sealer over the current precat coating. And then topcoat with another couple coats of precat. Should a lacquer based vinyl sealer be ok to spray over precat lacquer?

The questions you ask have no ready answers...as there is too much difference between "lacquers" from different companies these days......Some are very stable when stacked with dissimilar coatings, others give up right away and then there are those that only fail 6 months to a year out. The only way to be sure is to run long term test panels....or go back to a tried and true system.

I would not spray Simtec directly over any type of lacquer, precat or otherwise....I expect it will delaminate at some point. You could seal it with an Isolante. I do that sometimes when for whatever reason I cannot completely strip an old finish....My process would be to strip off the precat and start with white wood and your regular full fill process. Precat lacquer will usually come right off with some acetone and steel wool.