The cargo light was not intrinsically safe
nor explosion proof.
HK report is probably right,
but photos indicate lots of energy released.
Where did all that energy come from?
Must have been the thinner.
Why weren't the sprayers overcome by fumes
before they reached flammable concentration?
Probably the sprayers were working from the top down,
and the thinner vapor which is heavier than air
accumulated in the bottom of the tank.
This could also have been the area
with the least ventilation,
and the area most likely to be missed by the Mate's O2 checks.
Of course, since he wasn't supplied with a hydrocarbon meter,
there was no way he could really check for HC vapor.
The amount of thinner was at least six times
the max recommended by the paint vendors.
(There should be none.)
It is ironic that with this much thinner,
the coating life would have been very sort.