Blisters in the Paint

July 1994

Our thanks to Ian Wilde (Long-EZ G-BOOX) from England for all of the carefully researched material on this subject. I guess we are pretty naive about problems like paint blisters living here in the very dry conditions in the Mojave desert. Paint blisters are rare in our neighborhood and just about any paint system seems to hold up quite well. This is not at all true, however, in more humid areas of the US and, indeed, any other country including England. Ian reports having severe blistering problems over just about all of G-BOOX (except main gear legs and cowling).

He had the paint job done by an experienced aircraft painter in an unheated paint booth. Contouring was done with epoxy and micro and lots of elbow grease! Featherfil (a polyester material) was used as a "fine finish" over the micro. Corlar epoxy primer was sprayed over the Featherfil (allowing plenty of time for the Featherfil to completely dry as Ian was very aware of the hygroscopic nature of polyesters and he is adamant that this care was taken). The Corlar was allowed to cure overnight (per the data sheet) then wet sanded and allowed to dry. The sanding dA break through in a few places.

They did not spray any Corlar over these areas (a mistake, I believe - ED). DuPont's Imron top coat was then sprayed overall, all of this done in accordance with the appropriate data sheets. One wing blistered so badly that it had to be refinished within 6 months. The other blistered but it was 4 years before it required refinishing. The canard now needs refinishing after 5 years. WHY? The consensus from DuPont is that moisture was somehow introduced into the paint films. Apparently all paint systems have a process called "osmosis" which is the facility to allow moisture to pass back and forth through the paint films and no paint system is tight enough to prevent this process.

The possible sources of moisture suggested by DuPont are:

  1. Moisture contamination from the compressed air system. The compressor tank may need to be drained completely and, depending on humidity and weather conditions, should be drained several times a day. Water traps must be used in the airlines.
  2. Spray painting when the weather is bad - raining or very humid.
  3. Using the wrong thinner. The correct thinner must be used with each and every coat of paint, the primer, the primer filler, the top coat. Do not use one manufacturers thinner with another manufacturers paint, however good either may be. It is even worst to use a cheap quality thinner since these materials often have a moisture content well above specified limits.
  4. Flash point and drying times, as called out by the paint manufacturer, should be strictly adhered to. Many paint jobs are rushed, the painter thinking he is saving time but, in the long run, this can cause blistering. Applying a top coat too soon over a primer may not allow full evaporation of the thinner. This entrapped thinner will force its way through the top coat causing micro blistering and it may be months before conditions and temperatures are right for this to happen. GO SLOW, and follow the directions.
  5. An even paint film weight must be used. If you sand through a primer, re-spray the primer. Low film weights are one of the most common causes of blistering, especially when combined with adverse environmental conditions as described above.
  6. Contamination, such as salts (from finger prints) or from water containing minute quantities of salt, can cause blistering.
  7. Applying a solid wax polish to paint when it is still fresh should be avoided. Wax can seal the surface of the paint and trap thinners which can, in turn, lead to soft top coats and possible subsequent blistering.
  8. Finally, allowing the painted parts to "cure" in an area where there is high humidity can cause blistering later on because isocyanate activators are, themselves, "moisture seekers" and while not fully cured, can attract moisture.

What did Ian do to try to fix this problem? He used the following procedure: Sanded everything off the wing including the polyester Featherfil. He then applied a coat of wet micro and epoxy which was sanded to contour and, hopefully, would seal the wing. Corlar epoxy primer was applied, sanded and followed by the Imron top coat. The result: Five years later, U& blisters. (Careful attention was paid to all of the suggestions above).

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