I have a Keith Steele 1982 boat that I restored about 6 years ago (details and lots of questions somewhere on this site). I am going back to put a new coat or two of varnish on the exterior (I teak oil the interior). I noticed a series of cracks in the varnish, some of which are wide enough they catch on my fingernail.  They do not appear to have a similar mark on the interior.

My question is should I sand them down and built multiple varnish coats (I did 6 originally) or just apply my two varnish coats and let it refill the gaps.  These cracks showed up in the last year.