I've made an aquarium stand out of solid Ash and now want to clear lacquer the wood to finish it off. Given the aquarium is a salt water one what sort of lacquer would be best to chemically resist salt water spillages? I intend to also spray the finish on and am looking to achieve a nice finish, what's the best process to accomplish this? Thanks
You want a film finish for this and you have a few options here. The main two are varnish and lacquer, the others being epoxy and a pour-on finish as intended for bar tops.
Epoxy I think you'll want to discount first on the basis of cost, many times it'll work out to be the most expensive option by a considerable margin.
I think varnish is likely your best choice here since it's inexpensive, commonly available everywhere and can be applied equally easily to the legs as well as the top. Pour-on finishes are great but they can't be applied in that way to the legs, leading to a mismatch of look and surface finish on different elements of the stand which can be jarring*.
In terms of which varnish to use oil-based polyurethane varnish would be best for this if it's to take the weight of a water tank. Polys are more than waterproof enough if you build up a sufficient coat thickness, otherwise they'd never be used on commercial tabletops. Spar varnish is waterproof but it's made to be flexible to resist any amount of water and direct sunlight, which can lead to movements that would crack something less flexible. But that flexibility also translates into spar more easily taking impressions when subjected to pressure and how much does the tank weigh? In a thinnish coating you'd often not notice the difference but once built up into a proper coating, as you're going to want to use here to waterproof the top, a spar varnish will be noticeably softer than polyurethane.
In addition there's an aesthetic issue, spar varnishes are significantly yellower than regular indoor varnishes because of the increased oil content (this is what gives them their flexibility). Ash being a fairly light wood will take on a ghastly yellow tone that many find distinctly unattractive..... on the other hand your mileage on this may vary so ignore if appropriate :-)
There are numerous advantages going with varnish over lacquer, the first is the much simpler application. You can wipe varnish on and as touched on in a few previous Answers converting a varnish into wiping varnish is a simple matter of diluting it slightly and wiping it on with a rag (or brushing it on and partly wiping away the excess) and this application process makes getting a good result with varnish far easier, making it nearly foolproof. Additionally you won't need to buy any spray equipment and a solvent-rated respirator to protect you from the spray and fumes. And last but not least you don't need to figure out how to dilute the finish enough that it will spray well in your equipment, then learn all the other things that contribute to a good spray finish, and only then do one or more test boards since you never test out a new finishing regimen directly on the workpiece.
*On the other hand if this doesn't bother you then pour-on for the top and a waterbased poly for the rest might be worth considering.
Check out "spar varnish". This is formulated for use in marine envirouments (a spar is a wooden member supporting sails such as masts and booms). These, at least traditionally, are oil based. Oils not only repel water but, to my best guess as a Chemistry teacher, should not want to interact with other hydrophyllic substances such as salt.
This is also my choice for nice exterior wooden doors as it also resists deterioration due to the sun. I refinished our front door 25 years ago, it's still looking good.
Other type of oil finishes used for, say, teak decks and rails might also be appropriate.