by Kody » Wed Jan 22, 2014 7:33 am
Gudday Grant,
Hopefully I can help you out on this. I have build many boats and repaired them with Polyester resins and fiberglass cloth/mat. I have also used epoxy and Kevlar to build RPV's and repair F/G cruisers which were a great success. One thing is absolutely essential when working with epoxy resin. The resin and the hardener MUST be EXACTLY matched to each other. I have had the greatest success by weighing out the resins and matching the proportions by weight, within 1 gram. Any variation on this will spell disaster in big $'s. To mix the resins, pour part "A" into a container and part "B" into another container to get the weight correct. Pour the hardener into the resin and mix. Then pour the entire mixture into the container that held the hardener. Scrape every drop of resin out of the mixed resins to get the balance of the mixture correct. (These were the directions on the containers of the resins.) You can try what I do and add the hardener into the resin with the container sitting on a sensitive scale. Be very careful if you do it this way. Whatever way you establish the amounts of resin, be very accurate. It may be better to weigh out the hardener and then add the resin. One gram of extra resin will be ok but 1 gram of hardener extra can be a disaster. It is also very important to work at the temperature as stated on the container. Too cold and it wont cure, or take days to set. Too hot and it goes off while you stir it. You can buy hardener that will suit your requirements as to setting fast or slow but keep in mind the temperature is very important to also being critical as well. Epoxy is terrible stuff to get off your hands, arms, skin etc. The very best way to clean up is to wash your hands in cheap white vinegar. You will be amazed how effective this
works. If the brushes are still "uncured", wash them in vinegar as well.
Polyester resin comes in two basic forms, waxed and unwaxed. The unwaxed resin will "cure" to leave a very sticky surface. This is so another layer of glass cloth/mat can be applied and the resin will adhere to the sticky surface with no problems. When laying the final layer of glass the last mix of polyester resin must be a "Waxed" resin. This resin has wax in it that floats to the surface as the resin cures. The resin will have a smooth hard surface when this is used and the wax is washed off. I think I used acetone for this but wait until the resin has cured for about 3 to 7 days.
The correct amount of hardener (Methyl Ethyl Ketone Peroxide) to use with Polyester is no more than 2% and not less than 1%. More than this and the resin may develop serious cracks all through it. Less than 1% and the resin will not achieve the required strength. To ensure that the resin cures, the temperature is again very important. If you can maintain a minimum of 20 C (25 C is better) you will have the best cure and highest strength. You can use up to 3 % but be careful if the weather is warm to hot.
It is absolutely essential that the humidity be as dry as possible. I laid some polyester when the temp was around 20 C to 25 C and it was raining. The humidity was around 99%. This caused the resin to wrinkle on the bottom surface in the female mold and it was not nice. Fortunately, it was in a small area and could be ground out later and filled. The more hardener you add, the more the curing resin will shrink and develop cracks. It also gets very hot when the ambient temperature is above 30 C. Polyester resin will actually cure into a very hard resin using only 0.3% hardener. This requires a temp of around 25 C. I cast some solid blocks as 150 mm cubes using polyester resin and 0.3 % hardener. It may have been only 0.2%. It took over a week to cure but it did "go off" and cure to a hard block. Any greater percentage of hardener, and the blocks would have cracked as the resin would have shrunk too much. Clean up with Acetone, I haven't tried vinegar with this. Regardless of how you clean up, it's a messy chore.
Hope this helps out,
What Michael R Sydney has said is very good info.
Kody
Never be afraid to ask questions here, Prov. 11:14
