Working with fiberglass can be messy frustrating work for a newbie to the process.
'Fiberglass resin' as it is erroneously but commonly called, is actually polyester resin, and this can come in laminating and sanding versions. The sanding version has a wax like surfacing agent which rises through the resin after application and seals it which allows the resin to cure fully underneath and then it can be sanded.
With laminating resin there is no surfacing agent the the resin will remain ***** until it is covered. It will gum up sandpaper, it will not sand.
Polyester Finishing resin/Sanding resin can be made by mixing surfacing agent into laminating resin. Finishing resin can be applied directly on laminating resin, but finishing resin must be sanded before anything will bond to it. Laminating resin is used when secondary bonding of another layer of resin is to be applied after curing. Sanding resins can be used in place of laminating resins but Must be sanded before any hope of secondary bonding can occur. Sanding resin is not quite as strong and is slightly less flexible than laminating resin. laminating resin will remain porous if not covered with a layer of sanding resin, which also remains slightly porous on a microscopic level.
Polyester resins cure a bit less flexible than Epoxy resins, but they are more resistant to heat, and UV light, and are much less expensive. The MEKP catalyst/hardener is really toxic stuff. Methyl ethyl ketone peroxide bad shit. the rate of cure can be controlled by the amount of MEKP added to the resin. Too much MEKP can catch the resin on fire, and the fumes of polyester resin on fire are unpleasant and unhealthy in the extreme.
Polyester resins also do not bond nearly as well to wood as epoxy resins, and will not bond at all to some oily woods. polyester resins will eat many types of foam. A barrier coat needs to be applied to be able to use polyester resin on regular styrofoam and most types of insulative foam board.
Epoxy resins are much more expensive, and must be mixed exactly in the ratio prescribed, either by volume or weight. Miss this ratio by more than 3% and the epoxy will not cure properly with is full strength properties. Some epoxies have slow medium and fat hardeners. Some epoxies can allow these hardeners can be mixed to a specific cure time, others one must not mix the hardeners, but use the fast medium or Slow only.
Epoxy bonds much better to most all surfaces but for some plastics. compared to polyester resin.
Epoxy is more flexible than polyester resins and will stretch more before breaking. .
Epoxy will bond to and can be applied to polyester resins, but polyester resins 'might' not cure at all on top of a cured epoxy resin, and if it does cure it might not bond very well, relying solely on 'mechanical tooth'.( sandiing scratches) Any dings/dents, and the polyester can shatter and peel from the epoxy below it.
Epoxy can be applied to most any foam without dissolving it. Some brands and types of Epoxy can be stinky during curing, but some epoxies are quite mild in smell. Polyester resins will have everyone in the neighborhood downwind aware you are working with polyester resins, and the smell lingers in hair and clothes and seeming one's nostrils too.
Epoxy hardener is also nasty stuff. Some people can be hyper sensitive to epoxy hardeners, others can develop sensitivities over time. If one sands freshly hardened epoxy the dust is toxic, but fully cured epoxy the dust is inert, but fully cured epoxy is difficult to sand.
Tools used with polyester resins must use acetone to clean the tools afterwards.
Epoxy resins and one can clean the tools with distilled white vinegar as long as the epoxy has not cured. I prefer a mix of 91% rubbing alcohol and Distilled white vinegar for cleaning squeegees mixing stick/cups and gloves after application.
Acetone can be used with Epoxy for tool cleaning, but never never wipe uncured epoxy from your skin with acetone as this is an efficient toxic chemical delivery system directly to your bloodstream and a great way to develops an ugly sensitization to the amines in the hardener.
Epoxy degrades quickly in UV light, some more than others Epoxy will yellow much quicker in direct sunlight if not protected with a paint or other UV light blocker. Some Epoxy resins claim to have UV inhibitors built in, but these still do not last as well as polyester resins in direct sunlight. Epoxy needs to be painted/varnished, and many epoxies will have an 'amine blush' form during curing, and more so when exposed to high moisture levels during curing. This amine blush can be removed with water and a scotchbrite pad, and needs to be removed before painting, or adding another layer of epoxy. There are some 'blush free' epoxies that do not have issues with secondary bonding, or at least that is what they claim.
So, consider the facts above when deciding to buy some fiberglass and some resin and going to town. Removing a mistake is well more than twice the work before the second try can even be attempted.
I have launched various fiberglass working tools into low earth orbit followed by a stream of curses in the course of first learning to work with it.
Masking tape and clean cardboard all areas where you do not want resin, and clean cardboard itself can be used as a 'wet out table' where one wets out precut strips of fiberglass, squeegees out the excess resin, picks them up with gloved fingers and lays them in place carefully, overlapping in corners or bends trying to keep bubbles from getting trapped underneath.
The wet out table method is much less frustrating than cutting fiberglass, placing it in the desired area dry, then trying to saturate it in place, but that method has its time and place too.
All fiberglass conversion van roofs are made with polyester resins. The exteriors could be a gel coat like a boat, or painted like the rest of the vehicle. the inside of the roof is likely laminating resin without a layer of a layer of sanding resin painted on as sanding is not required on the interior.
I recommend using polyester sanding resin for use on conversion van roofs unless one already owns epoxy, or one really requires epoxies superior chracteristics like superior bond strength and strength in general, but of course they need to be painted and not allowed to exceed ~170F.
The fiberglass resins sold in autoparts stores is usually dark brown, and if one has no choice but to apply these in direct sunlight then overheating and too fast a cure will result, along with more a more brittle finished product. the best fiberglass resins are actually used on Surfboards, Silmar brand, but this is 2 to 3x as expensive compared to lesser polyester resins.
If you are in a area with Marina's nearby, try and get resin from them rather than from autoparts stores or from big box home stores.
Polyester resins also need to be tightly capped and kept cool. They are always in the process of hardening, slowly, and as such have a limited shelf life. The MEKP catalyst just speeds the process
Epoxy resins have a much longer shelf life, some say unlimited, but should be protected from freezing temperatures if possible. The Epoxy resin itself might turn all white and cloudy and appear thick if exposed to cold conditions over a period of time, but submerging the bottle in warm, not hot water, and not over the cap, will turn milky cloudy epoxy resin clear again with enough time and warm water.
My favorite epoxies are System3, but they are quite expensive compared to some other epoxies.
If one has their heart set on Epoxy, but is also on a budget:
http://www.fiberglasssource.com/sunshop/index.php?l=product_detail&p=88
System3 is 50% more expensive for their general purpose epoxy, their specialized epoxies like clearcoat/ Sb112 are 100% more expensive.
My least favorite Epoxy is West Systems, but they have the best marketing team and are the most recognizable name. They have strange mixing ratios like 4 parts resin to 1 part hardener or 5 to 1, which opens up more room for error in precise mixing. they also have a lot of Amine blush and secondary bonding issues because of it.
Most other epoxies are a simple 2 to 1 ratio resin to hardener which is much harder to thuck up.
The Apex brand epoxy above is OK, and good to very good for the low price, bt in my experience it does not have the superior bond strength of System 3 epoxy, and it is more apt to form a visible amine blush in humid conditions. Also when one is applying a final layer of Apex epoxy, and dust or surface contaminants cause the painted layer to be repelled and it leaves a mini crater or 'fisheye', which is unsightly, and a PITA to remedy, the easiest method of seeking perfection, is to sand the entire layer off and take extra precautions against dust and surface contamination for the next attempted application.
I was not even aware of the fish eye proclivities until I switched from System 3 epoxy resins to the cheaper Apex epoxy option.
There are another dozen brands of epoxies marketed toward the surfboard building market. I've not tried them all. these are generally made with a faster hardener to speed production, but then one has a shorter time to actually apply the resin.
One issue with both Epoxy AND polyester resins is they exotherm. They create their own heat when mixed. If left in a large mass like a mixing cup they will get super hot quite quickly and start to harden in the cup and start smoking and possibly catch fire. This is worse with epoxy, especially the faster hardener versions. The trick is to to get most of the epoxy out of the mixing cup as quickly as possible after a thorough mixing.
One other thing of paramount importance with Epoxy, is it must be thoroughly mixed, the Mixing stick must be able to get into the corners of the mixing cup, as well as scrape the sides. Do Not allow any Epoxy to escape the mixing cup when mixing, this will throw off the resin hardener ratio leading to an improper cure, the mixing stick itself must be scraped on the sides of the cup and then the whole cup mixed until one can no longer see any streaks in the epoxy. This can seriously take 5 minutes of stirring even with a mixing stick perfectly matched to the mixing cup.
