As this is an old thread, I thought that I would still chime in in case someone looks at this post and decides to do some fiberglassing. As an old fiberglasser, I can honestly say that is a crap job of fiberglassing, a lot of people go to Youtube videos and a lot of makers of the videos have no clue on what is involved and through their videos they just convince others to repeat their mistakes. I have been fabricating since I was 18 and the last job was 40 years later so I do have some experience ranging from making GRP propellers to building 35ft boats and a whole stack of industrial parts and moldings in between, a big part was repairing other peoples stuff ups!
The most common mistakes I see when looking at some of them is that they:
1) Don't prepare the surface properly, and unfortunately I have also seen this in some commercial GRP places as well.
2) They only use a roller or brush to wet out the glass mat.
3) They don't finish the layup with flowcoat or waxed resin. The difference between flowcoat and gelcoat is the addition of wax in styrene to the flowcoat. Putting any sort of resin, gelcoat or flowcoat over some previously laminated surface that has had a wax in styrene additive in it and that has not been sanded down to remove the wax and expose the fibers, will result in very poor bond which will delaminate over time, lose all bonding strength and enable water to penetrate between layers.
4) Not feathering the edges to remove hard borders in multi layer laminates. Mainly in parts subjected to flexing as in boat hulls etc.
The curing resin or gelcoat that is exposed to air will cause the surface to remain sticky due to moisture preventing the surface curing properly, you can laminate over curing resin only within a certain time overwise you will have to wait and then sand the surface down, which then becomes a very messy job and you will go through a ton of sand paper as that will clog up very quickly.
If you are going to fiberglass something, then you need a couple of additional tools to do the job properly. Ideally you want the least amount of resin in the matt that will bind the fibers and you need to remove all the air bubbles, a brush and or paint roller will not do this and the laminate will be weaker then it should be, you will also need some more rollers such as in the attached pics I have added to this post. Resin has limted strength and is brittle and you should try for a low resin to fiber ratio, in other words more glass to less resin, just enough to wet out the fibers and bind them to each other.
The first roller can be as small as 6mm thick, the middle one can be up to 8-25mm and the last ones are paddle rollers and usually between 20-25mm, the first one is for tight corners the second is for final rolling and the last set are for squeezing out excess bubbles and throwing/spraying out large volumes of excess resin, usually only used on large areas when laying up 2-3 layers of glass at once, the faster you work with those and the heavier you press, the further the resin droplets travel so be carefull using these. There also other types of rollers but the first two types will cover most DIY projects.
Now this post is not meant to be a "all you need to know how to laminate" as I would need to write a novel on how to do laminating to a proper standard and I don't want to come across as a know it all type, but I see these mistakes all the time. Laminating is a skill as it is in welding and there is a hell of a lot more to learn then just how to use a brush or paint roller to wet out the glass and call it done. We have all seen pics of bird poop in welding where someone with no skill has welded up something and thought that they did a great job, well the first two pics in the opening post is the fiberglass version of the same sort of crap. Not having a dig at "dobbsic" as that work was not his.
Just my 2 cents worth, hope this helps someone!
Cheers.
