I would sand it thoroughly then use a spray on grain filler, sand it back and then have it sprayed with 2pak polyurethane.
I had 2nd hand solid timber kitchen doors painted with 2pack polyurethane, they came up looking brand new, with a slight grainy pattern still visible, looked trick, and heaps of people commented on the finish.
I sanded them so the surface of the old finish was well 'keyed' (remove glossy surface) and smooth and consistent.
From what you describe, you would probably need to sand it all back to a consistent bare timber surface. Patches of varnish, and patches of timber will show through as an inconsistent quality of finish because the rate of absorption and drying time will be different. It s better to remove old varnish anyway coz there can be problems with poor adhesion when painting over varnishes and the edges of patches of varnish can 'fry up' from the solvents in new paint too.
if you like a grainy appearance just sand, prime and paint with polyurethane. If you want it smoother use a spray on grain filler, then sand and repeat if needed.
2pack Polyurethane can be mixed to whatever color and gloss level you want and is tough as. you can clean it down with metho or turps and it will withstand being outdoors to a degree. Timber is a natural product which means changes in the weather will make it move and shrink and expand which can cause cracking, particularly if peices are joined with internal 90* corners.
on a timber base, polyurethane can crack if you bump it enough to dent the timber
One question, and Im not knocking your Stag Chimpboy, what makes the stags special? I reckon they ugly
