INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Jobs

Paint thickness integrity difference between multiple coats and single coat

Paint thickness integrity difference between multiple coats and single coat

Paint thickness integrity difference between multiple coats and single coat

(OP)
Hi everyone,

To build up a thickness of 600 um we are applying 3 coats of 200 um of the same paint material, I suppose it's to avoid sagging during application but when on the first coat we achieve a thickness over the spec the client ask to sand down. I'm wondering if we achieve 600 um with a single coat if the integrity of the paint would be any different than with 3 coats, would we get cohesion failure on a pull-off test?

RE: Paint thickness integrity difference between multiple coats and single coat

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.

RE: Paint thickness integrity difference between multiple coats and single coat

(OP)
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.

RE: Paint thickness integrity difference between multiple coats and single coat

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.

RE: Paint thickness integrity difference between multiple coats and single coat

(OP)
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?

RE: Paint thickness integrity difference between multiple coats and single coat

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.

RE: Paint thickness integrity difference between multiple coats and single coat

(OP)
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.

RE: Paint thickness integrity difference between multiple coats and single coat

@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.

RE: Paint thickness integrity difference between multiple coats and single coat

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.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources


Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close