Though I haven't worked with that particular resin it can probably be painted. The main problem is getting them cleaned/prepared for painting as stated.
I would use a light 3M type brush/wheel, I would not use a wire bush unless it was for a smaller job and having a skilled operator. After brushing rinse well and allow to dry. After it appears dry get on the East or South side and dust or spray a little chalk dust, we use dye penetrant developer on a small area. This will tell you if you have any holes or voids in the existing tank that could cause problems with the paint. Wash the chalk off. If the surface on close examination appears to have any holes are voids prior to the painting preparation, I would try this test on a small area.
There are some materials available to clean the clean and prepare the surface for painting. I think most of the paint/coatings manufacturer have one and there are quite few on the market, especially at the larger marine stores, inexpensive to try.
The rust spots can be cleaned with any proprietary H3PO4 primer cleaner @ 100% concentration. Spray on let stand a few minutes and rinse off on small area at a time. As they say, to be determined by the operator. Watch out about cleaning with solvents if there are any holes or voids in the resin, they will trap the solvent the same as water.
You have the possibility of either using a primer for Polyester or as mentioned a high build Epoxy primer. If you use an epoxy keep it at the min thickness due to the tanks flexing. No matter which type primer you use stay with their brand of topcoat. I would trend toward a Urethane topcoat intended for surface exposure. Again as stated, I would talk to reliable paint supplier or two for their opinions and recommendations.
I also would contact the Scottbader people for a general if not a specific recommendation, then a general one like: use Epoxy, use Polyester, or not use either.
Here is one now two companies, in one Carboline and Hersite that we used a great deal.
We also use a lot of International, Henkal, and Dupont coatings.
The paint supplier is usually the one that has the best technical service.
The most important step is getting the right preparation with minimum surface damage and a good application that starts with a good applicator.
