Coating Reference Handbook - Hempel

HEMPEL
CHECKPOINT COATED SURFACE P 24b
BEFORE OVERCOATING
WHY?
Abnormalities in film formation of the coat to be overcoated may hinder adhesion
and correct properties of the coat to be applied:
* Spray Dust
* Exudation/Sweathing
* Holidays and pinholes
Spray dust acts similar to other dust, preventing or reducing adhesion causing
peeling and premature corrosion/fouling
Sweating/Exudation is the separation of binders or other material to the surface
of the applied coating. Consequence is loss of adhesion of the coat to be applied
and subsequent peeling and premature corrosion/fouling.
Holidays and pinholes cause lack of dry film thickness build-up. Also certain coats
are applied to obtain certain properties. Lack of these coats may influence the
final coatings behavior. Pinholes may blow their way through subsequent coats.
CORRECTIVE ACTIONS:
Spray dust must be scraped away and dedusted.
Sweating/Exudation may need a thinner- or a water-wash. However always
contact your HEMPEL-representative.
Holidays must in case of primers, sealers and topcoats be touched up before
overcoating.
For intermediates extra thick application of next coat may suffice to compensate
for lack of dry film thickness. Pinholes if very few are generally not considered
except in tanks. If many, ask your HEMPEL-representative for solution to the
specific case.
PREVENTIVE ACTIONS:
Influence application technic and shelter against heavy winds/ high temperatures
to reduce/ avoid dust spray.
Exudation occur normally only at too low temperatures, too high film thickness
too poor ventilation and/or upon exposure to rain/condensation too early.
Influence correct application conditions within the specified limits.
Influence application technic and stripe coating to avoid holidays and avoid
too low film thickness causing possible pinholing in next coat.
HOW TO DETECT:
Visually
Exudation often shows as a discoloration of the painted surface or a oily/
greasy layer on top of the coating.
INSPP24b, ed1 28/07/95 EMi