outboard primer - zinc chromate vs epoxy

Any opinions on this? i boat in salt water.

i've got the motor off my project boat to prepare to repaint, the lower unit removed, removing corrosion here and there. I'm leary of the canned self etching zinc chromate stuff (moeler,etc ) though i've already sprayed the lower unit with it. not touching the midsection that is all in good shape, but i have to do the trim/tilt, brackets,etc still.

Considering a non isocynate epoxy primer, but the ease of the canned zinc chromate stuff is tempting... If i decide to go epoxy can i shoot it right over the zinc chromate stuff i've already done?

 

well.....
you can get expoxy based zinc chromate
and powercoating works very well
Cleaning the salt out of aluminium is the problem

 

Hi Swade

What is yout objection to canned zinc-chromate primers please? I just swapped a Mariner 15Hp for a Tohatsu 8Hp because the former was too large and heavy for the new trimaran I'm currently building. The T runs very well, but the casing is a real mess - badly coroded. I'm considering using a needle gun then a wipe down with acetone to get back to good metal, but the question is then to use an epoxy primer and a polyurethane topcoat or use the aerosol zinc-chromate etch primer and finish with (auto) acrylic gloss.

Your comments most welcome

Cheers

Alan

 

Hi Alan, first I know very little on this thus the question,I have googled on this to see opinions, last time i did this i sanded scuffed sprayed the ZC from a can then used canned paint, didn't seem it was very durable lasted a couple of years pretty good though , but lack of prep no doubt contributed.

I've read that you can use epoxy over the zinc chromate. My understanding is the zinc provides the corrosion protection, the epoxy seals it in, then whatever your topcoat is.

Correct me if i'm wrong, i've gathered that it seems to come down to:

1) etching zinc chromate (canned or not)
2) epoxy primer
3) topcoat

vs

1) some etching wash
2) epoxy primer with zinc chromate
2) topcoat

vs

1) some etching wash
2) zinc chromate, alodine? etc
3) epoxy primer
4) topcoat

vs

etch or not (have read various things, etching even if with vineger they say will help anything bond better, though if you've sand blasted doubt that is needed).

1) epoxy primer
2) topcoat

epoxy with the chromate i'd imagine would be better than going over ZC canned primer (less chance of lifting) though i've read on several airplane and boat forums that is ok (check with paint supplier first) if you go over the ZC. I'm not sure performance wise how much benefit it has over just the epoxy primer i've read a few links on the web where studies mentioned they were comparable. Apparently zinc chromate is more difficult to find canned and in the eopxy primers these days though i can find the moeller canned stuff in my area.

I don't have experience with the needle gun but media/sand blasting bad corrosion seems to be the recommended thing, then immediately followed with either zinc chromate or alodine or immediate spraying priming. I had this lower unit blasted it was badly corroded and pitted and I had to fill in pits in various places with epoxy (marinetex i'm sure there are alternatives) i'm not sure anything besides blasting would do it, though I did read a few recommendations on the needle guns and was going to look into.

For the other pieces not as bad off I used one of those scuff wheels that removes paint, brass wirewheels, then sand paper over it and scuff the good painted surfaces since I didn't strip everything. I blew it off really good with the air compressor then wiped with acetone and immediately sprayed the zinc chromate. If i sat a few days, i resanded everything, again acetone,etc.

I'm going to follow with a light wet sanding of the ZC, epoxy + whatever topcoat. I've seen a few non-isocynate epoxies and paints that but i need to check with the paint supplier in my area as i know nothing about this, was hoping for some recommendations.

The canned stuff comes down to convenience I think, after cleaning whatever you have you need to protect it within hours apparently. So that would mean going ahead and epoxy priming , ZC, or alodine (? i think i've read).

I gave up and used the self etching canned zinc, i'll follow with a non isocynate epoxy and paint.

Also should mention using the ZC i'm very careful now that i know the hazards p100 dust and organic carbon filter when sanding or spraying, wore painters suit. Not sure of skin contact with the dust how hazardous that is...i wish i could have just found a small parts media blaster in my area that way nothing to deal with except the protect , epoxy, paint part.

I'd be interested in opinions from anyone who has done this quite a bit =)

 

G"Day Swade

Really appreciate your detailed response to my question. My own research indicates there is a potential problem with epoxy adhering directly to the aluminium unless the surface oxide is removed before a good bond can be be achieved - hence the needle gun and acetone.

I'm using West System epoxy, so I'll check their website, though they usually cover their bets and leave the final decision to the user. You are most certainly right though that the intermediate step of the Zinc Chromate will assist greatly. Therefore, the most likely technique to work will be:

• needle gun
• Acetone
• Zinc Chromate - this is available here in Oz from some of the aluminium fabricators
• Epoxy bog (mixed with microlite into a smooth paste and trowelled on)
• Sand to a smooth finish
• Epoxy primer
• Topcoat

I'm still not sure about the topcoat though I know poyurethane and epoxy are highly compatible. This combination (Altex brand) was applied to the hull of the 30' catamaran I built and apart from some minor scuffing, was as good as new after many years.

The reason I asked about auto acrylic in the last post is that I had success using this finish over lightly sanded 2-pack polyurethane. The merits of the auto finish are excellent coverage, vitually immediate drying and easy recoating. This was a much better option than redoing the 2-pack but I'm not sure how it will go with epoxy primer - more research is needed I guess.

Your comments about health risks are valid, but a decent mask seems to do the trick and it is not as though one is doing this for a living day in day out.

Thanks again

Alan

 

Hi Alan, what epoxy are you considering? I know nothing about these. Are you planning on stripping the entire motors existing paint? I called a paint supply today and they weren't really keen on epoxy going over anything (the zinc chromate) even though i mentioned it was self etching, DTM is what they mentioned (not sure if that meant dupont dtm or what) but it was direct to metal. I asked about going over existing paint and didn't get much info. I'd think epoxy over self etch zinc chromate would be ok?

But maybe it's because it's an auto supply? What they told me kinda contradicts what I see people doing on various forums.

Basically they suggested strip to bare metal, epoxy, + 2 stage. or if already zince'd and not completely stripped a single stage polyurethane over the zinc primer.

I didn't stip mine completely, mid section is perfect factory maybe 1 dime spot i cleaned..as long as that was in good shape doubt i could get better protection in those area. lower unit i had to strip, but other areas I just wirewheeled, abrasive wheel, sanded the corrosion areas.

Do you have any pitting?, i can tell you the sand blasting really opened those areas up for the epoxy (marinetex) i then applied to fair it back out. I wonder if the scaler will get in heavy pitted areas. When you do it post some pics i've been considering a larger compressor and one too.

Also i put my epoxy (to fill in pits,etc) direct on the blasted metal areas as it's made to bond with the metal...not sure if that was right, just followed the directions and some forum postings others had done.

- Shannon

 

But right i think I see your point, is the epoxy safe to go over existing stuff, followed with a poly, vs zinc chromate + existing finish go over with epoxy, then coat with acrylic?

I dunno let me know what you find out! I'm kinda halted atm trying to figure this out myself. Last time i sprayed something i don't remember it being this difficult but that was a long time ago probably laquer or acrylic was the choices then =)

 

Personally I wouldnt use a needle gun on aluminium as I could see it trapping corosion in.
Bead/soda/walnut/sand blasting or chemical clean would be much safer.

 

Tell your local Awlgrip/International/Alexseal/Sterling dealer you want to paint aluminium and see what they say for surface prep

 

Thanks "powerabout"

Good point about the needle gun. The pitting in the aluminium is very bad though fortunately, it is confined to the leg and and lower section of the engine case. There seems little doubt it will need filling before applying any finishing material so the dilemna is what to use for this. My thoughts about epoxy are based on extensive experienece with this material.

I guess I should have considered Awlgrip due to the good reports about it, though I have not used it myself.

Cheers

Alan

 

Does it need to be painted? its fashionable these day to not paint alluminium boats at all.

Alluminium doesnt like paint.

 

too true...
It comes from the sea and thats where its going back to....

anodising is easier and lasts just as long.
You can do bling colours as well

 

I have just spoken with the AWLGRIP technical rep here is Oz and am in shock about the complexity of the recommended technique, namely:

• Strip the old finish
• Apply 30Y94 etch primer
• Apply Hi-build epoxy primer
• Fair with AWLFAIR
• Recoat with Hi-build
• Apply 54545 as final primer coat
• Finish with AWLCRAFT 2000

Considering each of these products is available only in quart (1 litre) cans, this would not be a cheap exercise. Frosty may well have the answer, meaning leave it unpainted, but it doesn't seem a good idea on a material exposed to the marine environment.

I do take the point about avoiding the needle gun but unless someone has a better idea, I think I'll prime with the zinc Chromate, bog with epoxy, then coat with hi-build epoxy primer and finish with polyurethane. If it falls off after a year or two, so be it

Cheers

Alan

 

Thats just what we did when I worked in the megayacht industry.
99% of superstructures are aluminium.

 

Sorry - which of the above is what you did? The Awlgrip method, or what I'm proposing?

Alan