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I'm shooting behlens nitro onto a sanded to bare wood archtop (spruce/birch and maple)
It was recommended at the shop that I use behlen's vinyl sealer as my first coat or two, I guess particularly as I was thinking of having a light amber coat.

Is this necessary?
Recommended?
Bad?

I am far from being a finishing expert but I shoot sealer for two reasons. First, if I put anything on the bare wood to stain or color it I have been told I want to "seal" that before shooting anything that might draw the color back out. Seems reasonable.

Second, once upon a time I shot some colored lacquer over some maple without any preparation material. I did some staining to highlight grain, then shot a couple of coats of highly thinned lacquer, then the color over the top. I had a couple of places where the lacquer chipped off the underlying wood, mostly where I had drilled holes or was attaching hardware. I posted a picture here as I recall and was told I should have used a sealer to help the bonding process.

After that I have always shot a coat or two of vinyl sealer and I've had no finish problems. Knock wood...

Here is a bit of recommended reading for you, first a bit about sealers and what they do and why yoiu need one......https://howardguitars.blogspot.com/2017/01/sealers-for-air-dry-lacquers-by-brian.html

And a bit about nitro lacquer.
https://howardguitars.blogspot.com/2015/02/air-dry-lacquers.html

And here are my latest thoughts on nitro....
https://howardguitars.blogspot.com/2018/06/the-end-of-nitrocellulose-lacquer-era.html

_________________
Brian

You never know what you are capable of until you actually try.

https://www.howardguitarsdelaware.com/

Good stuff, thanks very much. Answered many of my questions and provided much food for thought.
I would be probably be inclined to use a water based finish (It's what I use for everything else(bows, guitar cabs what have you) but I had some vinyl sealer and a can of lacquer left over from the other restoration project I did a year ago. I will admit to enjoying finish that ambers over natural wood. Probably my only disappointment with the varathane diamond finish I often use in other projects. However it is a small disappointment when compared to the huge benefits of water clean up, quick drying and no poisonous stench.

I don’t find it necessary. I seal when I pore fill with epoxy. I use nitro and haven’t had a problem with adhesion. Vinyl sealer is not sanding sealer. It’s a moisture barrier.

Epoxy is your sealer when you pore fill with it...

Vinyl sealer is both a moisture barrier and a sanding sealer. Oh and a bridge to help prevent cold checks in the top coat.

_________________
Brian

You never know what you are capable of until you actually try.

https://www.howardguitarsdelaware.com/

I can't thank you enough for sharing your invaluable knowledge on finishing. If I were younger I'd definitely be switching to a modern finish over nitro but don't want to spend the $$'s since I don't know how much longer I'll be able to build. You are truly a valuable resource.