Painting #2, Epoxy primer / undercoats

[Guest Dry Reach]

Paint Problems #1 (Epoxy primers / undercoats)

 

After reading all the Posts today, while sitting in Sydney awaiting my next flight, i thought I would put all the “so called” experts (not you Wheels you actually do know what you are talking about Paint) into the picture.

 

 

 

In Order to help the general Layperson I have compiled this fact Sheet detailing the issues that can effect Epoxy Primers and Undercoats, and in Particular Fast Dry Hi Build Undercoats.

This is based on 25 years in the paint industry from a technical, sales, and “hands on” application perspective.

 

A. Background Info

 

Epoxy Primers and Undercoats are formulated / manufactured utilizing different types of Resins (base or Part A) and Curing agents / hardeners (Part .

 

The bases are many and varied types of Epoxies resins (durability, viscosity...) formulated for many application / usage scenarios and they are chosen in conjunction with the Additives (talc’s, calcium carbonates, silica’s…) required to make the product work (e.g. Anti corrosive, superior “surface wetting”, adhesion, sanding...) depending on what the product is designed to do. (Prime metal, tie coats, hi build, easy sanding…)

 

Differing hardeners contain differing curing mechanisms (products). Typically, Some are poly Amide, some Amine Adduct, and some a combo of each with or without a urethane.

 

Some of these curing agents oxidize with age in the can and go dark red and loose effectiveness

Some produce “amine Sweats” when cured that are very difficult to remove and are unseen by the eye.

 

 

B. How they cure

 

It is very important to understand the curing mechanism and curing process of epoxies to understand how problems occur.

 

Epoxy, once mixed and stirred properly, require an induction time for the products Hardener to react and “cross link” all the Base’s “epoxy” molecules together (too much hardener will “over link the product. To little will leave some product not “crosslinked”). Typically the induction time 10 -15 minutes but this will vary from product to products and also with Temp.

 

Once “induced” the product may be applied and may need thinning to aid application (pressure pot, HVLP, Cup gun…). With Airless it is best to not thin the product.

 

All Epoxies have certain conditions (temp (air and substrate), humidity (max), …) that they should be applied in. They also have “wet Film” build “minimums and maximums” they should be applied at and recoat times(minimums) betwen coats.

 

Also both components should be stirred independently prior mixing so as to not leave the solids behind in the tin.

 

 

C. Important!

 

The conditions they are applied in can greatly affect the cure time and thoroughness of cure. (example of this is - the difference between a Shaded side and a side in the sun)

When Epoxies cure on the substrate the time of cure is not only measured by the Temp and atmospheric conditions but also the film build applied (thick will be slower than thin). Also if they are thinned or reduced to aid application they will be even slower.

 

Epoxies have a curing window. This is a minimum (product cured enough to topcoat, but may not be cured enough to sand) and a Maximum (product is cured to hard to accept a topcoat.(even when sanded in some cases.))

 

Some Epoxies have a maximum recoat time that, when exceeded, will not allow top coating (urethane) to be applied until thoroughly sanded, cleaned down then a re- application of the undercoat followed by sanding etc … again.

Some have a maximum recoat time that will allow sanding, cleaning down, and then top coating.

 

Remember! These recoat “windows” will shorten and lengthen depending on TEMP and DRYING CONDITIONS as well as FILM THICKNESS and THINNING.

 

 

 

Now the picture is conveyed, let’s look at why, rarely, Top coats have issues with adhesion to epoxies. (Remember all epoxies are formulated differently which can produce differing issues)

Note:- Altex No3 was designed for high build, fast cure and easy sanding, thus it is more sensitive to people who “push” it’s limits and ignore the Specifications( smaller windows). Compare this to the low build, slow cure, poor sanding of the old epoxies like Epigaurd 199 (slightly larger windows)….

 

Technology has moved on to give people huge benefits! And these faster build, faster cure, easier sanded products “were” DEMANDED BY THE PRO’S AND AMEATUERS ALIKE! In order to overcome the constraints and costs of the older technology!

 

These products, like Altex 3, are widley accepted and used by painters the world over including Superyacht painters in europe. And these people do not tolerate rubbish or poor performance.

 

D. The issues

 

 

1. Under or over hardening. (this leaves a product with hard and soft spots which leads to people leaving it beyond the max recoat time to get adequate sanding of the “slower cure” soft spots.)

 

2. No induction time. (same as 1. )

 

3. Applying in humid, to hot, too cool, or poorly heated conditions. (This will cause the same as 1. but also can produce a “waxy” amine sweat, poor “through” cure, and too fast a cure.)

 

4. Applying uneven film builds. (as per 1. Plus over curing of low film areas.)

 

5. Incorrect use of thinners / reducers. ( overzealous use of thinners or incorrect type of thinners will Slow the product and leave the curing film open to many problems associated with temp, humidity and contamination), (if I had a dollar for every painter, including pro’s, who used the wrong thinner or brought cheap thinners such as MEK when told not to I would own a 100’superyacht)

 

6. Incorrect thinner type (reducer). (thinners are different to reducers. Reducers will cause failure in paint films if used incorrectly.), Using MEK can destroy the cure mechanism, the backbone structure and also cause Epoxies to have waxing and cure / hardness issues) USE THE RIGHT PRODUCT. (Using the wrong product is just like putting solvent onto plastic and watching it melt!... when it goes solid again it is brittle or very soft.)

 

7. Exceeding “max” recoat time. (This will make the undercoat too hard for the Urethanes “solvent blend” to get a “bite” into the undercoat and thus get the required adhesion. (Try painting any top coat straight on to Glass!))

 

8. Sanding with Zinc Sterate paper (this paper has a anti clog lubricant that upon sanding will soften from the paper (heat developed) and contaminate the coating)

 

9. Sanding with too fine a paper (I have personally seen many yachts sanded down with 400 - 500Grit paper. This will polish the surface and not give a mechanical key for the top coat.)

 

10. Poor sanding. (Missing areas will not provide the Mechanical key required for the top coat to adhere). N.B. – top coats need a mechanical and solvent ‘softening” key to adhere)

 

11. Incorrect sanding. (sanding in one direction will double the surface area. Sanding in a “cross hatch” method (and or random )will give 8 x more surface area and greater attachment for top coat.), (use guide coats to ensure every millimeter of the product is sanded and use the correct grade paper/s)

 

12. Incorrect clean down methods. (these include incorrect solvent and water soluble clean down products and methods, were either reactions or contaminates “infect / affect” the paint film. Also use of clean down application methods and tools that re introduce contaminants or fail to remove sanding Dust, amine sweats, Guided coat residue…), This is another topic I can explain at another time)

 

13. Poor “clean down” testing. ( a surface, with fine particulates of sanding dust and atmospheric dust is not clean until it “tested clean”. Ensure the surface is free of contaminants and use the prescribed testing methods to check this. Many don’t)

 

14. Correct thinning and film build of top coats. (Many topcoats have failed to adhere to the best prepared surface because applicators apply thin “tack coats” and let them set up “too long” before application of main build coats.! A thin Tack Coat has less solvent to “bind / adhere to the U/coat. It is also Under bound (to thin to form a solid film structure or cross linked matrix) and as such the next coat is adhering to a very weak film that will cause the topcoat to peel away!),( these thin tack coats are like putting a layer of fine chalk between the undercoat and the top coat. If you peel a loose piece of top coat off the hull and wipe the back of it with a black cloth and find a white substance on it’s underside then you may have this problem or sanding dust contamination.

 

15. Wrong solvent (thinner / reducer) used with Top Coat. (see 6. Above)

 

16. Spaying topcoat when Dew point (as measure on the Hull) is out of desired spec. (this is a big one to watch for in Auckland’s high humidity climate.), (best to paint in a warming, dry atmosphere than a cooling Atmosphere.)

 

17. Incorrect mixing of top coat and poor induction time. (as above but also will not give the topcoat the proper structure to bond and “clamp down / tighten” onto thr u/coat!. This results in poor adhesion to u/coat and peeling.)

 

18. Spraying in a environment were contaminants can land on the substrate post final clean down and pre final topcoat application. Many Boat builders have issues with this as opposed to “pro paint yards! (eg release agents, spraying in areas where sanding dust is entering the spray area..)

 

19. Low film build of top coat. (this will allow UV to penetrate thru the thin top coat and breakdown the Epoxy undercoat beneath (which is not UV stable.), A bit like not enough sun Block on your skin)

 

If you want me to elaborate then just ask. As it gets more detailed than this Laymans edition!

Cheers

DR

 

Well there it is and there is my call to get the flock outta here.

 

Sorry about the gramma and spelling BB but … boo hoo!.... I’m tired and need sleep!

[Atom Ant 0]

Well I'm no expert, but that was top f*cking class DR!

[smithy09 50]

Yup! Credit where credit is due... Good job DR.

[meandmine 0]

I am a professional marine painter. Ive been in the industry for 20years & think this is extremely well written. There are so many variables when it comes to preparation & application of marine paint, hence so many things can and do go wrong.

Well done on the post.

[wheels 543]

Squid, I reckon we need to have a "topic" heading in the Tech forum that is tagged to remain forever at the top. Whatever you call doing that. Then articles that are like this one can remain forever easy to find. If someone asks "how do I do...." then we can direct them to the subject.

 

Just to echo and add some support to DR's comment on mixing. I think this is one point often over looked. It is essential that any coating, but most especially any coating that has two components, is mixed thoroughly and allowed to stand for a set time. I can not over emphasize the importance of this. The best form of mixing is with a mixing machine, but we amatures and even some pro's don't normally have them. A stirrer on a drill is not always the best means, as it does not always get the very bottom of the can, so just be aware of that and ensure you use a stick to get right to the bottom and around the edges. If mixing solely by hand, use a lifting method in the stir to ensure you lift the contents from the bottom of the can to the top. Stir till the coating is uniform, with absolutley no streaks of anything. Some coatings mix easily, some are a real bitch.

Then if the instructions say stand, stand for the required time.

[wheels 543]

Seeing as we are talking Epoxy or two component coatings and seeing as this is a good technical discussion. I thought I would add a comment about Amine Blush.

Amine Blush is a waxy substance that can occur on the surface of some Epoxy coatings, given the right, which is wrong, type of conditions. Interestingly, it is actually an issue with a component in the Hardner used by some Epoxy manufacturers, being a cheap and easy reactant. It reacts with Carbon Dioxide and moisture (major problem for upper North Island) to create this waxy substance that migrates through the curing mix to form on the surface. This can then result in subsequent coats of undercoats and top coats being contaminated as well and may also result in poor adhesion to the Epoxy. The "wax" (it's not actually wax) can continue to migrate through any of those additional coatings and cause a yellowing of the top coats, not to mention, affect their correct curing.

Removal is simple. Yet often incorrectly carried out. The Amine Blush is water soluble, so the best method of removal is by warm soapy water.

NEVER use solvent and NEVER sand it. It's a very weird stuff in that it can be worked into the cured Epoxy surface by both of those methods and then once you apply your undercoat, it migrates back up through the undercoat and into both that and the next lot of future coats. There is a special Two pot undercoat that can be used to block this condition should a mistake like above have been made.

You can hopefully reduce the chance of Blush by applying Epoxy in warming temperatures and as dry as possible Humidity. Try not to Epoxy late in the day as the temperature is falling, which is when moisture starts to come back out of the air.

Also, never use an LPG heater to warm a shed, as this creates too much moisture in the air.