The first thing to look at is the manufacturer's recommendations in the product datasheet. Then take a look at this document and perform a word search on 'thickness' to see what can go wrong when control of application thickness is lost:

http://offers.exova.com/paints-and-coatings-failur...

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/pub/8/83b/b04

All answers are personal opinions only and are in no way connected with any employer.

I understand the failures due to over-thickness, but I would like to understand the difference between 1 coat of 600 um and 3 coats of 200 um on the same product.

If you understand the failure modes then you should understand why there is a restriction on film thickness. Does the manufacturer's datasheet explicitly state that a single coat DFT of 600 microns is required for optimum performance, not forgetting that for such applications as tank lining, multicoat systems will help to reduce the incidence of pinholes.

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/pub/8/83b/b04

All answers are personal opinions only and are in no way connected with any employer.

I do not argue about the specification and why there is a maximum thickness, cracking, sagging, pinholes, bad adhesion etc... I'm just wondering if you get the exact same thickness in a single coat without any visible defects, why to sand down?
My guess would be that we could get cohesion failure and that the integrity would be altered during curing.
but how does the curing is any different when it's overcoated the following day? as full curing is around a week in this specific case.
Why to sand down and the next day build up again?

Good question, one which only the client can answer if the remedial action for overthickness was not specified.

SSPC PA2:

Correcting for Low or High Thickness: The specifier should specifically state the methodology to correct the applied and cured film for low or high thickness. If this information is not contained in the specification, then the coating manufacturer’s instructions should be followed.

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/pub/8/83b/b04

All answers are personal opinions only and are in no way connected with any employer.

SJones answered your question.

TTFN (ta ta for now)
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Actually I don't have any issue with the spec and for the application, everything is fine but given my lack of chemistry knowledge I'm just curious about the chemical reaction/process. The paint supplier representative on this project doesn't know much as well.
If the spec require 3 coats when it's apparently possible to apply it in one layer or 2 then there is certainly a reason, so far I thought it would mainly be to avoid sagging.

Again, SJones pointed out that pinhole prevention requires multiple coats.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
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@geothai79 as already discussed, there are numerous failure modes that can be attributed to excess thickness dependent upon paint type and formulation. Since you are not at ease with the specification, your next point of reference is the paint manufacturer's datasheet that gives an indication of the optimum DFT per coat. If you are still not at ease with that, contact the paint manufacturer and ask to see independent test reports of coating tests that demonstrate satisfactory performance and the DFT giving such performance. Indeed, the sanding of intermediate coats might be considered a little draconian, but the motivation of the client is not open for speculation given the paucity of information.

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/pub/8/83b/b04

All answers are personal opinions only and are in no way connected with any employer.

There are many reasons for applying paints in thin coats.

Alkyd paints cure by reaction with oxygen. Thick coats will not fully cure and will skin over and wrinkle.

Water based paints require that the water evaporate before the coalescing solvent does. A thick coat will trap water in the coating preventing the resin particles from coalescing.

Surface tension helps to level and smooth coatings. Thick coating will experience variations in solvent and surfactant concentration during the drying process, which will cause orange peel and other defects.

Gravity causes drips and says.

Paints shrink considerably during drying, which will result in defects in thick coating but not in thin coatings, such as mud cracking.

The science behind paints and coatings is quite complex and varied.