again not sure if this would work, but sounds good lol. Copied and pasted from a forum.
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There are two issues involved in painting polyethylene. One is to "activate" or "modify" the surface chemically to allow wet out and adhesion. In industry there are numerous methods used to treat the surface of polyethylene before bonding or painting. The best are too hazardous for a small shop or garage since the chemicals involved are actually dangerous. Reasonable bond strength - enough for paint - can be achieved with very light surface roughening using detergent water and fine wet-or-dry sand paper. This must be followed by careful flame treatment. An oxygen rich flame is best and the flame must not be allowed to melt or glaze the surface. Instead, the farthest tip of the flame only should contact the surface and then only for a brief fraction of a microsecond. The flame must be kept quickly moving at all times. Every square inch of the surface must be touched ever so briefly by the farthest tip of the flame in this way. After some cooling, the surface can be retreated if in doubt - but I would suggest running the flame in a cross direction to the initial hit.
The surface can be tested with super clean distilled water. Apply a single drop with a very clean eye dropper. On an untreated tank, the drop will bead up like on a well waxed car. No adhesion. If properly surface modified, the drop will actual cling to the surface and spread out to be at least twice as wide as it is high. This gives adequate adhesion for most applications.
The second issue is to use a paint compatible with the flexibility of polyethylene. I suggest using a flexibilized paint or adding a flexibilizer as is customary for flexible bumpers. All the major paint suppliers offer this. Of course, a flexibilized clear coat can be put on top.
As to priming - I feel it just interferes with flexible performance and adds extra work. If the tank needs scratches or dings filled in this may be possible if you can find a sufficiently flexible filler. In any case if you were to sand the primer or the filler down and even lightly hit or scuff into the polyethylene, that area has to be re-flame treated. If you hit it hard enough to bubble the primer or filler, you are way overdoing it! Several light quick sweeps with the flame will do the job - a near-scorching will ruin it!
If you need optimum adhesion or structural bonding for repairs or modification that can be done too but the chemistry has to be kicked up a lot.
