Primer directly over tung oil, and poly - no prep
 

I am building a franken fender and want to know if this plan will turn out reasonably ok.

The body is an el cheapo that somebody used a tung oil finish on 'plus a light poly gloss'. To tell you the truth, I just want to give it a couple of coats of grey primer and that's all. Should I sand off whatever is on there now (whoever did it did a terrible job) or can I just spray the primer over it ?

Obviously I'm being lazy, but I don't really care to do a ton of sanding this time around (been there, done that many a time).

if there are no dents or deep scratches that could cause drips or relief in the surface i guess you could just spray the primer.

I would sand it up to 400 grit before spraying

+1. And use an epoxy primer.