Firstly the fact that many producers of GRP tanks use the cheapest possible materials and production methods, is something that means the quality of many composite tanks leaves a lot to be desired, both in terms of chemical resistance and durability/strength! Today even though the effects of ethanol bearing fuels on GRP are well documented, some producers here in the UK continue to use inappropriate materials and production processes, with many seeming to think that use of a chemically resistant resin system, will result in a chemically resistant tank!
Secondly the use of high cross link epoxy resin systems as tank sealers is highly questionable, as while these materials have a high level of chemical resistance, they are very brittle when cured. This combined with the fact that uniform film thickness of the resin after application, and proper preparation are both very difficult to achieve, means that such products can only be looked upon as a temporary solution to fuel tank problems.
Those who produce the various tank sealing products seem to have entirely ignored the fact that in service a fuel tank will be subject to high and low levels of resonant vibration, and that this in combination with less than perfect preparation and in most cases widely varying film thicknesses, will result in sealants cracking with failure occurring relatively soon afterwards in many cases.
Before the introduction of alcohol bearing fuels, if it were possible to prepare interior tank surfaces to the appropriate standard, then polysulphide type sealants which retain a certain degree of flexibility when cured, seemed to present the best possible option, but while such sealants are still appropriate for use within the aircraft industry (with no alcohol bearing fuels), they are not resistant to long term contact with alcohol type fuels.
At this moment in time it seems anyone with a GRP tank can only be 100% assured of it being fully resistant against Efuels, by sectioning it, then properly mechanically abrading the interior surfaces, and applying a chemical resistance layer comprising synthetic surfacing veil, a novalac vinyl ester resin with a flex agent added to all surfaces in contact with fuel, bonding parts together using the same novalac VE resin as a base, and finally appropriate post curing.
Some of the currently available tank sealers here in the UK fail after long term immersion tests using E85 fuel as a control, others based on high cross link epoxies are fully resistant to the E85, but as outlined earlier the brittle nature of these materials, will generally mean failure is likely to occur at some stage if they are being used on GRP tanks, which are much more difficult to prepare than steel, and tend to vibrate much more.
A product which was able to incorporate the positive points of polysulphide and novalac epoxy, would provide the ideal solution to problems with tanks, but as none of those producing tank sealers at the moment seem able to properly test them, or provide any sort of industry standard chemical resistance data, at the moment its very clearly simply a case of which one provides the most credible advertising blurb, and hoping for the best when pouring it into your tank!