I've just finished a bit of a diorama, with a heavily worn looking U-176 returning from an arduous mission in the Atlantic, passing an old S-Boat alongside, and watched by a couple of officers and dockworkers.
Hi, thanks, the technique is simply very dark rust for the small actual rust points placed strategically in positions on the model, and light rust streaks where the water has run down and stained the paintwork. I've been practicing weathering on u-boats with the Tamiya kits and appropriate paint,but it sure doesn't look that good.
I think the best thing to do is try various techniques, find which suits you best, and keep practicing. There are many brands and types of resins available today just as there are different types of reinforcements for different applications, so how do we know what to use?
The other thing to consider is "is it worth repairing?", this is a very sound question we need to ask ourselves as some items (like flares from 4WD to front fiberglass spoilers on sports cars) could be an expensive and time consuming affair ending in "it would have been far cheaper and quicker to buy a new or second hand one and get it resprayed (to colour) if needed.

Fiberglass materials in general have to meet a countries standard of quality and assurance, so it will be a personal choice of what kit you would buy.
On the other hand if our fiberglass canoe or object is older and show signs of gelcoat or surface breakdown of the finish then we can paint it in a 2 pac or a single pack finish depending on our budgets and what we have on hand or whats available to you.Some objects like repairing a fiberglass wheel arch flare on a 4WD or sports car will be better off spray painting the whole unit, more so if the colour is a metallic or pearl colour as this will blend in and will also save time cutting in and polishing it to match the surrounding area.
So if we are not going to be using a gelcoat as our final colour then we will need the following materials to finish our fiberglass repair.A small tin of car body filler that is suitable for fiberglass and that will go under 2K paint (2 pac), a cork or rubber sanding block, some sandpaper of 80grit, 150grit and 320 grit, a spatula, paint scrapper or a plastic disposable bog spreader and a clean plastic lid from a 2 litre ice cream bucket or something flat and clean to mix some filler up on and some clean cotton rag. So it is also a good idea to look at other areas around the car,  boat, canoe or the object that we will be spraying cause it is on the cards that there will be some left over, so if there are any other areas you find it would be beneficial to prepare those areas as well so they can be touched up. Here's a preview of a few small photos, before submitting to the gallery.The U-176 is from Mirage Hobby, a nice kit which is very finely detailed, and the kit comes complete with all photo-etch. Note, not much rust on the S-Boat as their hulls were mainly of wooden constructionThe light rust can be applied using either thinned paint or a good technique is using the Tamiya weathering sets which are widely available in hobby shops, also some dirt streaks are added in addition to rust streaks in a similar way.I've re-hosted the photos to provide larger images of the close up photos taken previously, which are easier to see.

I did this 'U'-boat partly as practice for some heavy rust action, I'm quite pleased with the result, but still learnt lots for next time. Well for most, they just go out and buy a Fiberglass Repair Kit and for most of these people that will do the job.So, what else do we need?
The S-Boat is from White Ensign Models, again a nice kit with plenty of photo-etch detailing, and the figures, bicycle and staff car are all by L'Arsenal.

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