Restoring an Old Spam Can -- To Strip or Not to Strip

What's the weight penalty for not stripping an old 172, just painting over
the old paint, after sanding it first, of course. I'm new at this but
willing to make a go of it. I'd like to use a high-gloss polyurethane and
some bright colors. TIA for your advice. I've been receiving a lot of
priceless advice in this forum lately, and in a yahoogroups forum. For
which I am grateful.

BTW, the top done on the O-300 has been wonderful. Millenium cylinders,
rocker arms rebuilt. The engine has around 8 hours and hasn't even used
any oil. I am impressed. The rings are seated now and she has all kinds
of power. Well, at least for an O-300. After flying a Taylorcraft the
flaps are fun too. And so is flying at night.