Can anyone that has experience with gel coat give me advice on using it to repair stress fractures and chips that are not hurting the structural strength of my fiberglass canoe. Why can’t I just use bondo? I have heard of marine tex however I already purchased the gel coat.
Thanks for any and all advice.
First things first.
You should know a little about the type of gel coat that was used on your canoe. If the manufacturer used polyester, that simplifies it some. Even if they used vinyl ester, you can use polyester, but it might depend on how extensive the repair is and how fussy you are.
Did you get gel coat the color of the canoe, or are you going to have to fiddle with coloring the gel coat?
So you said that the underlying layup is sound–that’s good. Remove all the loose chips and sand the sharp edges of the gel coat around the damaged area.
You will need a working temperature above 60 degrees when you apply the gel coat. Just before you brush the mixed (mekp added) gel coat to the wound, wipe the area
with acetone. Try to brush on enough gel coat so that the repair area will be higher than the original surface. There will be some shrinkage, so you might have to make an additional coating, before you get around to finishing the whole thing.
With the gel coat applied, cover the repair with waxed paper. Tape it down tight and let it cure. In warm enough conditions the cure might only take a couple of hours, but don’t rush it. When it is cured, check to see if the shrinkage will require another coat. If so, wipe again with acetone and brush on another coat.
With the proper thickness added, tape all around the repair area, so that you do not sand on the areas adjacent to the repair. Use about a 500 grit paper on a sanding block to carefully begin to smooth and shape the repair. You should be able to move up to about 1000 grit, then switch to 1200 wet sanding, then 1500 and 2000. Don’t be discouraged if you find some imperfections; just mix a little bit more gel coat and add where needed.
When the wet sanding is completed with the 2000 grit, it’s going to be pretty good, but you might want to finish up with some polishing. Remove the tape from the surrounding area and buff the whole thing out using a fairly mild polishing compound. Wax and you’re ready to go.
Chips and gouges you can fill easily
but I wouldn’t waste time with those fine spider cracks. As you say, they are of no functional significance, and repairing them to a good standard is difficult. The time is better spent paddling.
Whether such cracks form seems to be a function both of the flexibility of the underlying layup, and of the flexibility of the gelcoat. The gelcoat on our old Moore never showed fine cracks. It was over a thick, stiff layup. My Mad River Comapatriot had an excellent flexible FG layup, but the overlying white gelcoat developed lots of fine cracks. Later, the gelcoat developed hydrolysis blisters. But on autopsy, the underlying FG layers were not hydrolyzed or delaminated. The problem was only with the gelcoat, and not, in my judgement, worth fixing.
Because I don’t care about appearance, I use high quality epoxy to fill chips and gouges.
My real solution to gelcoat problems is, I don’t buy composite canoes that are gelcoated anymore. An outer layer of pigmented S-glass is harder and stronger, and easier to fix.
prior post
search: First gel coat repair attempt = not good
Today, the stern keel area of my Solstice Kevlar Titan was repaired with West Marine Scratch Patch Gelcoat. Search the title in ‘shopping’ for other branding of the product.
Goes on with considerable finesse from the tube mix, not my mix or hand control.
My take on Gelcoat, unsubstantiated, is gel absorbs impact thru shattering, as a not too bonded cover for the layup especially at the keel.
The repairs were small pocks where the coat sheared off.
I noticed scratches at the very stern when finishing so spread Gel there finding the mix went skimmed over
scratches, filling the shallow gouging but leaving a very thin trail on the ungouged gelcoat finish…using a cutout polybottle spreader.
I would use a toothbrush for cleaning, one for acetone, one for detergent, then heavy water rinse.
I used Turtle Wax sticker remover(Wal) wiped on with a paper towel or two then Meguire’s red bottle car wash scrubbed in with a toothbrush.
Suggest pressing Gel into pocks and running the spreader plus more Gel over the pocks…pack Gel in so the goo gets under and into the original coat, anchoring the new Gel into the pock or scratch.
Shock absorbing through shattering
I think what you are actually observing when gel coat cracks that way, is its lesser ability to flex than the hull material itself. Since a thin layer of gel coat has virtually no flexural strength, it really can’t absorb enough energy to contribute to hull protection when it flexes due to impact. The fact that it cracks only shows that it is brittle, or at least that it is brittle when in the form of a very thin layer (and no one wants a thick layer of gel coat on their hull).
Gelcoat is a good ablation layer for
the keel, for light to moderate use. Our canoes, however, get heavy use in rocky, gravelly whitewater rivers, and gelcoat on the keel isn’t going to be there long. That’s why I’d rather start with exterior S-glass, the hardest cloth available for composite boats. It also contributes very well to hull stiffness and resistance to damage, especially if backed up by inner Kevlar layers. While carbon is stiffer and stronger, carbon wears away readily. S-glass does not.
SO, if one doesn’t mind the weight, and isn’t going to abuse a boat, gelcoat looks nice, absorbs UV, and provides a wear-away property.
impact energy dissapation
try searching, for example:
gel coat impact energy absorption dissipation
Try reading
– Last Updated: May-19-14 12:19 PM EST –
The first several articles that came up with those keywords are about structures that are far different from canoes and kayaks. I didn't read the details but when I see such things as super-high-strength ceramic armor and very thick exterior coatings of soft material, I know it doesn't apply to canoes and kayaks. I have no time to search and read a bunch of that stuff but did find this, in an article about recreational powerboats, within a paragraph stating why one company doesn't approve of the use of gel coat:
"In order to develop a more flexible structure two key aspects of conventional boat building have changed. First is the elimination of gel coat. The issue with gel coat is that it is the structural weak link in the composite system. Its very low strain-to-failure and its location on the outer surface combine to make this the first material to fail. Gel coat cracking is problematic.. To put this in perspective, more than half of all recreational boat warrantee claims involve gel coat cracking. An emerging technology we have trademarked “SharkSkin†is a cost effective alternative to gel coat for use in marine structures. The material is a tough elastomeric polyurethane, polyuria hybrid similar to that used in truck bed liners, dredge pipes and even Navy composite rudders."
Note the part that said "Its very low strain-to-failure and its location on the outer surface combine to make this the first material to fail." Doesn't that sound like it's exactly related to the character of the gel-coat layer that I described in my post above? So tell me, how does a paper-thin layer that has virtually no resistance to bending/breaking contribute in any meaningful way to energy absorption when it DOES bend/break? THAT's what I'd like you to explain, or provide a link to, rather than just making note of the fact that the material CAN be useful in certain specialized applications that aren't related to canoes and kayaks.
Also take note that some of the toughest composite canoes you can buy, such as Millbrook, do not have gel coat. Surely they would have it if they thought it contributed to impact resistance.
datakoll, we’ve said all the limited
good that can be said about gelcoat, and energy absorption is not one of them. It’s a dead issue. Olympic whitewater kayaks need all the energy absorption they can get, but no maker wastes one ounce on gelcoat, or any other surface coating. The surface is S-glass or carbon mixed with resin.
If you want to google around for a structural justification for gelcoat, go ahead, and bring us the results. We won’t hold our breath waiting.
?
Covered bottom scratches with West Marine one tube gelcoat this afternoon. Hot and dry around 90 with breeze.
The one tube gel IS well mixed. using a poly bottle cutout for spreader and surgeon quick swipes, a scratch cover is almost thin enough to ignore sanding. Definitely in the ‘the hell with it’ category.
The mix shrinks. A true gouge or pock with OEM or? coat impacted off the hull thru energy absorption will take 2-3 apps before you get to file it down to spec.
The mission was filling 2-3 keel pocks and then covering scratches with a protective coat not cosmetic hiding.
Use the tube not store to harden.
Very quick and easy to use. Recommend.
you ought to publish a book
There’s so much good info on your website. If nothing else you ought to get more for your efforts.
Actually, I do have a book, but…
…not on that subject. http://greenlandpaddlebook.com/
I have a lot more information that I intend to post on my personal site, but I just haven’t gotten to it. It used to all be on Webshots, before they went belly-up.