[Steve Neul]

Yes and no whether it could be used or not. The spar varnish really needs to penetrate into the wood and if you seal it with shellac it would prevent that. It would work but you would shorten the duty life of the spar by doing so. Usually it's a good idea to thin the first coat of a spar so it gets deeper penetration.

 

[Steve Neul]

Technically you can add an aniline die to a varnish to recreate the effects of amber shellac. I know it would be alright for inside work, I just don't know how it would affect the duty life of an exterior finish. You might try it and stick it out in the direct sun for the summer and see what affects it has. The color could also fade to clear. Aniline dyes are prone to fade.

 

[Steve Neul]

All dyes and stains fade in sunlight; water-soluble aniline dye fades slower than alcohol or oil based.

I just meant the dye may disappear in a couple months in direct sunlight where shellac would probably remain the same or darken.

 

[Steve Neul]

Some stains like minwax are prone to fade but certainly more colorfast than a aniline dye. I haven't noticed any measurable fading with any other brand stain. I've added cabinets to a kitchen before using varathane stain many years after the initial job and they matched. Outdoor finishing is a lot different story though. An aniline dye would fade under artificial lighting but at a much slower rate than outdoors. What was discussed is suspending the dye in the finish which makes it more susceptible to fading.