Below is written some years ago when was I a little more angry but is still how it is with pros when time = money and discerning ( anal ) customers are involved.
How to touch up a minor surface scratch.
For a solid 10 years while developing my paddles and boats I supported myself by repairing high end sailboards, racing yachts and all kinds of paddlecraft. Many times my shop looked like a tradeshow except the boats were not all glossed up. I have seen boats that were bailed out of, the owner finally putting her life in perspective, not wanting to go up against a rocky shoreline and 6-8 foot surf. (Read more in materials) Or the TWO fiberglass tandems that simultaneously pulled the rip cord at 85 mph.
As most repair guys will tell you, it is most amusing to see the faces of those who sometimes think there's nothing left but the crying'. What they will NOT tell you is how easy it is to repair. The mystery behind the curtain is how they make their money. Read grossly overcharge. The fact that I never played on these fears is probably why I still have people calling me years later to let me know how great their repaired to like new equipment is holding up. If I am fortunate enough to be good at something it is probably diagnosing and repairing these thin laminates and maybe building paddles and boats. I intimately know about what works and what doesn't when it comes to kayak and paddle construction. I have been accused of preferring to build boats and paddles rather than use them. I will only say you honestly get your money's worth when I build something for you. My 'loss' is your gain. Thank you.
How to touchup a minor surface scratch or gelcoat gouge. Taking the above exultations to heart, this is how I would do it if I really wanted to have something come out nice and not cause further trauma. If you do not make a mess you probably are doing it right. Alot of this detail work can also be applied when touching up the paint on your car. Be patient, check your work, and use tape until you get a feel for things.
What you need: Most things can be purchased at Home Depot or a Marine store. Or ask me.

• Acetone for clean up. Maybe.
• Rubber gloves
• Dust mask or better yet a respirator for the dust AND fumes.
• 3/4 inch masking tape, generic brand is fine, save your money for more important things........
  *.....Like sand paper. Imperial Production wet or dry sand paper from 3M is worth the extra cost in time/effort /boat saved. 320grit, 600grit, and finally 1200/1500grit...2000 if you can get it.
  These numbers would most likely be 280,400/1000/1200 if you can only find the generic brand. These are the approximate equivalent of the superior 3M. I am already writing a book here but I need to say something about some of these (non 3M) sand papers lest other composite guys call me on it. Some of these papers have silicone in them and may cause bonding problems on subsequent repairs should you need to do something over. Just call me if you need a more elaborate explanation.

*POLISHING not rubbing compound. As above please read label and avoid silicones if you can. Label should also say "For removal of 1200 /1500 grit scratches." I use 3M FINESSE IT because it does a great job, washes out of non-skid, clothing, and I can relate to the name better than Great White.

• Buffer or any thing that turns quickly with the aide of electricity and will accept a quality buff pad.. People who do this for a living do not have time to waste on one of those wimpy sloth buffers. Buy a quality buff pad at the same place you get the compound. I prefer the hairy wool POLISHING pads for gelcoat and boats. If you will use this on your car (paint) more than your boat you may wish to get a foam pad as these work well and are safer to use on all the contours cars have. P.S. you really do not NEED a buffer, a little elbow grease will also work with compound and a clean cotton rag though not nearly as good. Three minutes hand buffing will not bring up the shine 3 seconds with the tool will.
• Utility razor blades...lots of uses.
• Flat file..maybe
• Flat 2"x3" piece of something that bends a little bit. Do not go for the '3"x6" 'wetsand block ' next to the paper .......too big for scratches. My favorite is .250 high density foam. I can even break off a corner for tiny spots.
  Bends a little bit= 1/8 paneling, rubber truck bedliner, dumptruck mudflap, cd case (cd is too flexible), or a piece of cardboard matching the footprint of a block of hardwood. Or get the block and cut it down. The control you get with the smaller size far surpasses the speed and potential for mistakes you get using the block full size.

*Material; if you are lucky you have some of the original gelcoat that matches your boat perfectly. (Supplied with all TIDELINE kayaks) You may contact the manufacturer and request some, have the boat's serial# at hand. I highly recommend doing this. If you care enough about your boat's cosmetics to be repairing scratches you will NEVER be satisfied unless the color is at least close. Now that I think about it , a severely scratched boat would look good if the scratches were repaired in a complementary contrasting color. While I am here I will say this. Most people I have ever talked to are overly worried about scratches. I always try to talk folks out of fixing them.. they really are not harming the boat. Each scratch is it's own entity and ALOT more work than you think to fix it properly and if not properly than why do it at all? Look at the bottom of your boat, which ones do you start with? There are probably alot more than you thought. I honestly think a scratched/non-repaired boat looks far more attractive than a cobbed, patched up one. If you are getting ready to sell boat maybe polish it up, but that is really as far as you should go in my opinion. Why not just re-gelcoat or paint it??? Please read answer in future FAQ.
Should we stop here???
If you cannot get the proper gelcoat here are some options;
Use any color, try to use those generic pigments to match your boat (good luck and over use of these inhibit the complete curing of gelcoat), pay someone to match it ,i.e. how much money do you have?...Buy a new paddle with this money instead, or use clear-slightly thickened resin. I feel the latter is the easiest to work and live with.
I may raise some dust here but the clear polyester FINISHING resin you can buy at Home Depot or surf shop/possibly model shop probably not at the marine (laminating and not pretty) store dovetails right into the plan. It's clear and will only slightly darken original color, no fussing with color and it cures fast and consistently . If you wish you may use straight resin to fill scratches, gravity may want a say here and your answer to this is to tilt boat or use a bit of filler to thicken the resin. If you are really going for cosmetics, take your time and use straight resin. Otherwise purchase the smallest amount of any kind of filler. West Systems 406 is perfect.
MAKE THE TIME TO DO IT RIGHT
Do:

• Soap and water boat. Dry.
• Identify scratches you wish to repair. If you really are going to do this ,go for the ones that are through to glass first. Not necessarily because these need it but these are the deepest and will give you a taste of what's to come and if you want to stop, at least you have solice in knowing you got the big ones done.
• If you have a Dremel tool or die grinder ever so slightly rough up the scratch starting at the middle and working towards the edges on either side of scratch, depending on the bit you are using (I like the egg shaped ones) you may be able to do this all on one neat pass. Just clean it up a bit. Use your judgment whether or not to go deeper when cleaning out chipped edges. This whole procedure can also be done using a small round file. I use (and in no way avocate you do the same) a razor blade held perpendicular to scratch in two hands dragged up and down. P.S. I traded in the noisy die grinder and dremel for this.
• Contrary to everything you have or may read; do NOT take this opportunity to wipe the area 'clean' with acetone.....it is as clean and fresh as it can possibly be and to wipe it with acetone risks smearing surrounding area's contamiates back into it or glazing (my word) the surface over. Blow it out or use a small clean brush.
• Once you have cleaned this up tape along edge of repair leaving a tiny (1mm) gap next to cleaned out area. This allows for material lifting a bit when tape is pulled but not out of repair and leaves a bit of sacrificial material to aide when fairing things in later. Tape as continuous and flat a line as you can. If you are messy use paper here to mask surrounding area.

From here on resin and gelcoat get treated the same way.

• Set boat so resin will run as little as possible if you are using it neat. Use tape to make dams if you want knowing it is just more sanding later. Mix up resin fairly 'hot' and work in small batches. Thicken to slightly thinner than peanut butter and use a fresh razor blade or cut down plastic applicator to trowel mixture into scratches pushing a bit and going in two or more directions to get a good bond and to be sure the area is 'wet out'. Hopefully your tape job is good. Do not go too far away from these as you will now use a clean blade set flat on your smooth tape job to drag excess mixture off and thereby cutting your sanding time down to almost nothing.
• Pull tape or not???????? I will always pull tape straight up unless mixture has started to cure. I do not recommend pulling tape if you are using resin straight. I do it but you have to babysit it and wait for the time where resin will not pullout of repair but will pull up with tape. You may have to do a second run with the straight resin...do this as soon as first batch of resin kicks in. Do not worry about sealing wax.. just keep going.
• Once tape is pulled and resin has started to cure I usually push my luck a bit and trim the high spots down with a razor blade held flat against contours of boat. Beware of pulling repair out or cutting too low and your fingers here.
• Whatever you do please DO NOT fall for that "place a piece of wax paper over repair" there by smearing your repair all over boat and having to do it again piece of misinformation. The object is to have your repair be ever so slightly proud to which you blend it in with surrounding area without going too low. This procedure guarantees doing things multiple times especially if surface is convex. For small scratches you really do not have to worry about top layer of gelcoat curing hard as you most certainly have a fully cured layer just below the top of your repair and will buzz right through that top layer effortlessly. If you want. Purchase some PVA from your marine store/composite supplier with which to seal (brush on) top layer right after gelcoat has gotten to a rubbery state......or spray it on once gellcoat has just started to kick in.

*If I am really feeling lucky I endeavor to use razor as a scraper and bring filled area down to almost perfect. Then I START wetsanding at my final grit. Try it and save time precious or.......................walk away for a few hours-overnight.

• You should have no more than playing card thickness to sand down..narrow strips no less. Warning :the gelcoat is very thin on kayaks/canoes. Thinner and less forgiving still on cars. (Paint) You really do not want to even touch (too much) the area surrounding repair until the last few strokes of 600 grit or higher. If you are not confident or just want to play it safe put tape back down and start wet sanding and changing tape as it peels up. Now I hope you can see why that bigger block would be a hindrance. If you left tape in place (neat resin) keep going until you can peel tape up. If you really had left alot, the file or a coarser grit might save a bit of time and chew through the heavy stuff faster. Keep dipping the paper in water to keep a sharp 'edge' and check your work often by wiping and inspecting. Wetsand the entire repair a little at a time.
• If your repair was barely sticking up (fingernail just catches) you should be starting out with no coarser than 600 grit. Resist the desire to sand all around the edges of repair...get the middle down a bit then work towards the edges and finally from the surrounding area into the repair using your smooth, held flat piece of flotsam. If you start to see a dark color from below I would either stop or move right to 2000 and call it a day after that. Things are getting thin. Or switch to a finer grit earlier to be safe. In my experience you will not get as perfect a repair doing this as the finer grits do not take off much material (they tend to simply round things over) when using a flexable pad....... but what you have will end up shiny.

Speaking of shiny, do you really want to have a bunch of glossy spots on the bottom of your boat? If not wetsand only to match surrounding area's sheen and buff. Or remove all the fittings, keep going with the 2000 and buff out the whole boat to have it look better than ever right as you sell it.

• Wash off all wetsanding sludge. Dry. Install buff pad on machine. I like to put the compound on the boat first ... why load up the pad before you even touch the boat? Use the "Haul out the chainsaw once a year to decimate flora" throttling technique. So compound does not centrifugally vanish, gradually increase speed as compound drys. You do not have to get every last bit off with buffer... it will wash off later. Satisfied? Wax it. I could go on and on here with subtle tricks of the trade. If you come to a point where you do not know what to do please feel free to ask.