By your description Tom I'd guess like the previous respondent that you're attempting to thin oil based varnish with water which, as you've found, doesn't work.
The solvent for oil based varnishes is white (mineral) spirits. You can also use naphtha which flashes off more quickly. All you are doing by adding a solvent is reducing the amount of solids you can apply to the piece of work which is often useful, but there are always checks and balances required in thinning.
Almost any finish is suitable for ash, but it depends on the intended use of the end product. For instance, linseed oil is an attractive finish but I wouldn't use it on a dining table as it has almost no
resistance to liquids and none at all really to moisture at a microscopic level, and wet glasses leave a white ring. Linseed oil finish requires a slow long winded application regime and is easily repairable if damaged, it darkens the wood on application, and it continues to darken over time. If this is the finish you want then it's suitable.
Pre-cat lacquer is applied with a spray gun in three or less coats, takes about three hours to apply all told, dries to a hard film, protects the wood, doesn't change the wood colour as much, prevents gross liquid spillages getting to the wood, is durable and fairly hard-wearing. When it does eventually fail it's not so easy to fix as linseed oil. You also need a good extraction system-- spay booth ideally-- to go with your spray gun, a spray mask and some other protective clothing along with specialist paint filters and materials handling bits and bobs (COSHH regs in the UK.)
Varnish isn't much fun to spray without a spray booth or perhaps spraying in the open air. Problems with spraying oil varnishes include the very slow cure time which means the overspray remains liquid and sticks to everything it lands on, including you. Runs, sags, and curtains can happen all too easily if too much is applied, and the slow cure means there's plenty of time for contaminants to get into the wet film causing blemishes and extra work. But once applied it's durable and prevents stuff getting to the wood until the film breaks down whereupon it's a bit of a pig to repair.
Water-borne varnishes dry quickly and one of these might suit you, but you'd need to check that your gun has stainless steel or plastic parts because without them rust can build up quickly ruining your gun-- incidentally, I had no idea that Bloody Awful & Questionable sold spray guns and as far as I'm aware they sell tools that can only marginally be described as suitable for woodworking.
Back to the varnishes, I do still find all water borne varnishes so far to be somewhat ugly being rather cold and dead looking when dry or they have a bit of a blue or milky cast to them. They can be warmed up by applying a sealing coat of de-waxed shellac as a first step, and shellac does impart an attractive hue to wood.
De-waxed shellac can be used under any other film forming finish. Slainte.
