I know this sounds pretty radical but paint weighs alot, anyone ever built a minimax or himax and flew it without any paint? I wouldn't leave it out in the sun too long.  

Maybe too radical?  Paint seals the cloth and makes it air tight.  Use a thin coat of clear dope and the weight will be minimal.

I had posed this exact question in homebuiltairplanes.com  ( Used id keith103 ). Here is the link:

http://www.homebuiltairplanes......ighlight=#post295892

When i painted my skypup I thinned the latex down with water. Empty weight was 195 lbs

This issue was very important for my build. I am hoping to make Pt 103 with my build, and skimping on the elaborate painting process is an easy way to make weight. The other option for weight reduction, from what I have heard, is to install a Hirth F33 engine, which is much lighter than other comparable engine options. Downside being it vibrates like crazy, from what I heard. No personal experience with ultralights yet. Never flown one.

Oratex.
Just don't complain to me about the price.

Early CFM Shadow microlights (Ultralights to you guys) struggled to get under the (then) 150kg limit, so they were given just a coat of clear dope. Worked fine.

From my experience with polyester cloth in the sun, even the most expensive marine sail fabrics which have special formulations to reduce UV damage must be covered when not in use or they rapidly degrade.  Even if one is very careful to cover the sails, a white sail of many times the weight of ultralight covering fabric will degrade to the point that you can tear it with your hands in a few years.  Some cloths are deliberately made very heavy and of very dark color to extend their life.  I think the safest trade off here would be either a fully painted fabric to get the maximum endurance or the Oratex which is light but not inexpensive.  Although much more expensive than latex painted fabric you might want to evaluate the costs compared with fabric painted with something like the complete PolyFiber system.  Perhaps it might be more cost effective in that comparison.

Tom

It appears that the goal is to meet a very specific weight requirement and not to save money.  Oratex may be the lightest....I know it was used to cover a plane that the owner was willing to spend any amount of money to make the absolute lightest possible ( http://www.betteraircraftfabric.com/success-stories/lil-cub.html ).  I expect that very light dacron fabric with one or two thin coats of clear dope would be similar in weight to Oratex and much cheaper, too.  Except that you will have to recover the plane much more often if it spends much time in the sun.

Quoted from Keith103

This issue was very important for my build. I am hoping to make Pt 103 with my build, and skimping on the elaborate painting process is an easy way to make weight. The other option for weight reduction, from what I have heard, is to install a Hirth F33 engine, which is much lighter than other comparable engine options. Downside being it vibrates like crazy, from what I heard. No personal experience with ultralights yet. Never flown one.

Just a fyi, I bought a used hirth f33 for use on my skypup and didn't get many hours out of it before the cylinder head bolts stripped out along with the spark plug threads. my assumption was I had the belt redrive pointed up and and it blocked the cooling fins overheating the motor over time weakening the engine. I installed a rotax 277 and never had another problem.  http://machnone.com/jeff.html