I just purchased a used SEDA GYPSY fiberglass kayak. The deck (and hull) has several scratches. Looking for advise improving look, minimize appearance of scratches, etc. Can minor scrathes be buffed out? Products, claners? I applied regular car wax with minimal results.
sanding…
Depending on how deep the scratches are, you could do a little wet sanding with 1500-2000 grit sand paper. You dont want to do too much and go through the gelcoat though.
A gritty buffing compound and fine polish will get the shine back.
Good advice from kyakmaker
Although I ususally use 600 grit wet sandpaper. If the scratches are deep, you can fair them in with something like marinetex. Add a pigment directly to your filler before you apply it and you do not have to worry about sanding through or touching up paint.
fg scratches
"I just purchased a used SEDA GYPSY fiberglass kayak. The deck (and hull) has several scratches. Looking for advise improving look, minimize appearance of scratches, etc. Can minor scrathes be buffed out? Products, claners? I applied regular car wax with minimal results."
Just use it. Scratches means it's getting used and that's the point. That' what gelcoat is for--it's a sacrificial layer. If you want more 'sacrificial layer' get a Brit boat. (Uh, oh.) Don't worry about 'em unless you see that they've gone through to or are close to going through to the fg itself. Then, get some gelcoat, learn how to apply it ( http://www.wavelengthmagazine.com/2005/as05gelcoat.php ) and go have fun and learn more skills!
In the process of learning to apply new gelcoat, you'll learn the skills and obtain the equipment to improve the look of those minor scratches if you must. 303 will make the smaller scratches almost go away as far as looks go and it should be applied regularly throughout the season anyway, at least to the deck of you boat (and other sun soaked gear besides waterproof breathables) to protect it from the sun.
Marine Wax
Try a marine wax instead of car wax.
But really, use the boat, enjoy each “beauty mark” that you make as a momento of another great paddling day. 
at least…
My advice is don't sand it. Instead of a narrow little thin spot in the gel coat, you will have a big thin spot in the gel coat, and it will probably stand out more than the scratch did and illicit comments like "did you fix that yourself?".
At least you won't have that sinking realization when you know you just put that first raker in a pristine new boat.
The first glass kayak I bought was a well used rental boat, a Current Designs Solstice GT. If using the term "well loved", it would have looked like a 12 year old's security blanket.
I tried all the tricks and techniques that first summer, and none of them did anything to improve the look of the boat.
What a great boat though ! I still have it and it has taught me not to worry about scratches.
I paddled that sucker into places I wouldn't have dreamed taking a shiny new kayak and had better times because I didn't worry about it. It also tought me how durable a kayak is too !
Don't sweat it, go paddle someplace new and adventurous. If it helps, think of the money you saved and smile :^)