Mark, 2 component Poly cant be over 1 comp varnish , now there is regular 1 part Polyurethane and this one is compatible with regular varnish,
a) Can I put Varnish over Polyurethane without a devastatingly bad result?
YES YOU CAN, NO PROBLEM.
b) Can I put Polyurethane over varnish without a devastatingly bad result?
SAME,, NO PROBLEM IF THE POLY IS 1 COMPONENT. 2 PARTS
POLY SOFT AND PEEL OFF 1 PART VARNISH.
c) Can I put 3 layers: Polyurethane: Varnish: Polyurethane?
IF IS MONOCOMPONENT YES...
To bad you cant strip the old coating, i mean if you dont want to take the risk , with a heat gun and a wood scraper is a piece of cake for me , 4 months agoo we strip all the cabin sole
, take 1 week, and i use west system , 3 coats with the thin hardener to seal the teak
sole , and after 4 coats of Perfection 2 part varnish , the result is amazing , and i hope last for many years....