MIMF

Hello, I'm returning to applying a nitro cellulose lacquer finish to a guitar after a year or so has passed. A problem with the top revealed itself during the original finishing process and I put it away as the humid season was approaching. I'm back at it now and have sprayed the top which had been stripped. The back and sides are already built up and fairly level but could use a couple of more coats. I'm concerned about the new lacquer melting in with the old and am wondering if some kind of prep coat or special mix would help promote a positive result.

Thanks, Denny.

This is traditional nitrocellulose lacquer?
If so, and it is only a year old, then you should be able to skuff it up with P600 (or so) and the new finish will bite into the old.

Yes, this regular nitrocellulose lacquer.

It would be wise (but not absolutely necessary) to use the same brand of lacquer as you originally used on the guitar. I second the recommendation to scuff sand the finish before spraying a new coat. Also, make sure the surface is very clean. A wipe down with a clean rag and some naphtha is highly recommended.

This morning I remembered that I had a test board that I sprayed around the same time last year. I found it and just finished sanding it down with some 600 grit wet and dry. It's not quite as level as the guitar but should be ok for another test. I am using the same brand of lacquer. I'm glad that you mentioned that Barry, as I was thinking of switching. Thanks guys.

I usually scuff sand and clean the surface and just before spraying lacquer, spray a light but wet coat of thinner on the old lacquer, then give it a few minutes to evaporate and soak in before the new wet lacquer goes on. You need to spray enough to make the surface wet but no more or you risk runs and sags. You can also use retarder to soften up the surface of the old lacquer, as it gives a longer time to work on the old surface because it evaporates so slowly.

I got a little ahead of Michael's recommendations on the practice board I was able to find. After sanding with 600 grit I sprayed a normally prepared lacquer wet coat and after drying the 600 scratch marks were highly visible. I then sprayed a coat of thinner with retarder as I recalled the Dan Erlewine's "flash coat" video from last year. The methods Michael and Dan described (very similar) worked well and the scratch marks could not be seen. So today I'll give it a try on the guitar. It should go ok if I can overcome the jitters and don't start spraying as if I were Zorro.

Seeing scratches after spraying one coat is not indicative of a failure. It just means you need to spray a few more coats.