Rash Guards aren't going to provide you with too much warmth, and in some cases with my experiences, make me colder for some reason. Some guy on yahoo answers said ,
"Rash guards are designed to dry quickly, and not hold the water, so they don't weigh you down. That being said, however, wearing wet clothing in the open air accelerates the rate at which body heat is lost. Therefore, you will not only feel colder (cooler?) but actually speed up heat loss and risk hypothermia. Buy a cheap wetsuit its really worth it."
Well, can I still give you info even if I'm not from southern cali? In my opinion, you should just wing it and get a 3/2 if you want a rashie that will give you as much warmth as you are looking for. I don't have a neoprene type rash guard, but I think a 2 mil. one would work best. Probably with rubber welded seams. Wouldn't your legs still be cold though? Might as well just get a fullsuit or a spring suit.
(I was typing before roar did, so I didn,t know he was going to give you this info.)
None of those are going to do any good for what you want because they're all permeable. You need neoprene. O'neill RG8 1.5mm shirts go for about a bill. Rashguards keep your nips from rubbing off and the wind off your back.
Where are you surfing that you're trunking it? I was in a long sleeved spring suit on Friday afternoon and my toes got chilly. The air was 84 but the water was in the high 50's. The air temp dropped back into the 70's over the weekend and I've been in a 3/2 since.
Marshals- get a under armour shirt for 15 bucks. but make sure to get the one with the fleece on the inside- polypro
if you got the cash, get the neoprene something, it will be warmer. at least get a cheap generic one for a 100- i dont remember the water being that warm down there... but then again i live in jersey- you know- the overrated state that is on all the covers of the surf mags... ever heard of jersey??