Good way to seal cracks in fiberglass?  

ideas
Fiberglass is generally hard to fix - I've used some clear nailpolish for minor chips. Bondo will patch a larger crack - but keep in mind - you are repairing, not restoring. A repaired chair will never have the same value as a good condition chair.
Its also okay to leave the chair as is - a good patina is appreciated in vintage furniture. Good Luck!

thanks for your suggestions...
thanks for your suggestions and insight... appreciated! 🙂

There's a Product...
...for repairing surfboards called SunCure, you can get it from any surfshop- basically it's a kit that comes with a tube of fiberglass paste, some non-stick smoothing sheets, and some sandpaper.
You apply the paste with a popsickle stick, smooth it with the sheet, and then place the surfboard (or in your case the chair) in direct sunlight for like thirty minutes. Sand, and you're done. The only problem is, it dries somewhat yellower and less transluscent than fiberglass laid down in strips- so you'd have to paint. If you don't live near any surfshops, here's a link to a place to order it:
http://www.wavejammer.com/surf/fsurf.htm
Another suggestion would be to take it to a surfboard repair specialist- the fix all sorts of awkwardly placed dings and cracks, I don't see why they couldn't do a chair- and professional repairs are generally flawless and invisible- they're real good at matching the texture and color of the original fiberglass... most ding repair places get from 30 to 75 bucks depending on the size/location of the damage...