prep, prime/paint and fair bottom on aluminum boat

Hi Folks,

I have searched this forum, found and read some recommendations for prep/ blasting primers, etc, but still have questions.

I will need to apply antifouling to the bottom (5083- will be in salt / backing tidal river for 5 months peryear) and before doing so will fair some warpage from excessive heat from welding (topsides have no distortion, but I got carried away welding the bottom stringers....). I am not concerned with bottom looking perfect but figured it would not hurt tp fair low spots with filler, while I am at it anyway.

1. I do not have access to soda / sand blasting and have read some do prep with sanding disks/ angle grinder - Is this OK amd if so, what recommended grits?

2. Some mention acid washing and others just say to blast or sand to prep... Do I need to acid wash, and if so please recommend product and sequence, i.e. before or after sanding or in place of sanding......

3. After prep, please recomend product(s) that I need to apply, in order / sequence! i.e. do I need to prime first and then fair or juts all at once..if so, with what products / brands?

4. Once fair, which bottom paint to use?

Thanks for any help! Aluminum boats are rare in my area so not alot of local knowledge / experience at my local Marine/paint supplier.

 

Interlux/ International paints have an excellent website. Available across Canada and US from Akzo Nobel

Choose the paint and then follow their recommendation
You cannot use a copper base antifouling on aluminum boats though I do not think that anyone makes a copper based antifouling paint anymore
We usually just did a SAND, acid wash and a primer and then a two part epoxy paint.

Did the bottom pull up into the boat due to stringer distortion of are the areas between the stringers just warped, ie the panels between supports

 

Hi Barry,

I would say a little of both. I went with thicker bottom hull sheet than plans called for (3/16" vs 1/8" in plans) but smaller stringers (only 1 1/2" x 3/16" flatbar aft (I went with angle more fwd and had no issues there. ) The flatbar moved somewhat as the heat distorted the hull between or main frames. There is not give or movement if I step on either the high or low spots on the hull or stringers, so I figured I would fair the low spots and live with it.
I realize a pro, or even me with more experience would have prevented it, but at this point I just want to progress to get the boat in the water at some point. I made a much more conscious effort in welding the topside stringers and had juts a bit of print through in a few spots but very little if any distortion.

What would you use for fairing material and do I use it right after etching and primer? I have experience using different strength/ type fillers in epoxy (west systems) but that was used over bare / ground fiberglass. I have never used fairing compounds with other systems / paints/primers,etc....

 

We have never used fairing material so cannot offer any insight, perhaps the Interlux people might know
Regarding the print through, there is a 7 inch sanding disc that has 1 inch holes cut into the disc and with corresponding holes in the stick on sanding disc, you can easily take care of the print through and watch through the disc as you sand it out. Normally we would use about an 80 grit or even a 60 grit to smooth these out.
I would not try to use a hard grinding disc to smooth the print throughs as it takes a lot of know how to do this even on a flat surface without under cutting the metal and then with shiny paint, the under cut shows badly.

 

Being under the boat is a blessing so far as cosmetics are concerned, faults on the topsides are far more obvious. Plate boats are usually painted, and many have a surprising amount of bog to hide flaws. Unless you are a speed demon, a few minor undulations won't make much difference to the boat's performance.

 

No, It will not be a speed machine, so not too concerned with small efficiency gains. But, if I have say sanded, etched and primed it, and I can spend an additional few hours applying a bit of epoxy or other strong filler to the worst area or two (and then sanding fair) I figure it will be worth it.

I checked out the Interlux website and it looks like there are a couple of potential primers that could be used after first sanding and following their recommendation in applying "Primewash 353/354" which I assume to be an etcher... But, upon then not being able to find Primewash 353/354 on their site, I then read in a (several year old) forum post that it was discontinued years ago...yet it is still recommend in the prep section on their site.

So, I am still looking for a solid recommendation of which (commonly found) etch agent to use, and then if primed after is it OK to juts add medium to high density filler to west system epoxy and fair away? If so, would I then need to prime again over the sanded / faired spots again before applying any of the recommended antifouls?

I can get a bit of a discount on Petit and some other products, and I see a few choices like:

- fiberglass and aluminum dewaxer/wash from aquaguard

- self etching primer 60-316N

- Petit metal primer 6455 self etching primer

- several antifouls for use on aluminum and other hulls in albative and regular

 

What size is the boat? The stringer size suggests a small hull as you went from 1/8 inch plate to 3/16 inch.
IF the stringers are straight then perhaps the way to fair the plates would be to flatten the plate by adding in a few more short stringers or cross pieces and push or pull the unfair portion into a flat profile
Do you have pictures?

 

Hi Barry - boat is 25ft. I did try jacking down on the stringers to push out the "sunken" or "starved" areas, between a couple main frames, but the stringers just moved sideways, or twisted, as they were flatbar. I guess I could have added a couple more, using tee or angle, which may have pushed out better.
I may try tacking on a more rigid stringer material, tee or angle, and try jacking down on that to see if it works better.
If it doesn't make much difference, I think at this point I will live with it and just fair if possible.
Don't have pics of the areas and the boat is an hours away.

In addition to product and sequence questions above, or perhaps related... would it make sense not to simply prime after sanding and etching but to coat instead with say a West systems epoxy coat, and then use epoxy and filler in the worst areas? Would that give a greater bond strength than applying filler/ fairing compound over primer paint?

Finally, I think I read somewhere (Kevin Morin I think) the suggestion to etch even the topsides which will be unpainted.. Is this a common practice and if so, what is the reasoning? The boat will be upside down when I do the bottom so I can see that alot of it would run down across the topsides anyway.... I assume it would be best to sand entire topsides lightly before doing this as right now topsides are a mix of perfect shiny material, some scratches and the sanded portions where I butt welded two sheets together...
Any more advice would be appreciated!

 

In an earlier post I mentioned 60 - 80 grit for sanding. This would be for an area that is will be painted, not an area that is being left in it normal state.

I would test try 100 but you might find that it fills up the sand paper quite quickly

Re I do not see any advantage in etching the topsides if there will not be paint on it. The etch mainly gets rid of the aluminum oxide. Not sure why you would want to etch the entire non painted surface
I would really stick with one paint manufacturer and follow their process. Interlux must have a etch, primer finish coat adhesive system. Phone the Akso Nobel for info

 

OK - here is where I am at - have two general choices based on info found from sources thus far:

Approach A:
1. Sand
2. Use Aluminum Cleaner such as Dupont 225S or similar
3. Rinse with water
4. Use Etcher such as Dupont 226S or Alodine 1201
5. Rinse with water
6. Prime
7 .Paint with antifoul


Approach Baccording to Interlux tech support)
1. Sand with 80 grit
2. Wipe with de-natured alcohol and dry with cloth in other hand
3. Apply 1st coat of Interprotect 2000E within an hour of cleaning, thinned 15 - 20%
4. While still tacky, apply 2nd coat of 2000E
5. While still tacky apply 1st coat of antifoul - Trilux 2
6. after 1st coat dries, apply 2nd coat of Trilux 2


For fairing (as recommended by West systems tech support and agreed by Interlux support):

1. After above sanding, use Alodine on recessed areas and then apply West systems 105 with fast hardener for low temps, and use either low density filler 407 or first apply more structural coat of codical sillica, keeling a bit low as not easily sandable and then follow with coat of low density filler for easier sanding.



I am leaning towards approach B as finding and buying Interprotect 2000E is easier than other chemicals and less steps. Their rep swears the 2000E takes care of all the stuff that other cleaner/etchers do and it will adhere as good (he says better!!) than separate use of cleaner, etcher and primer....
But, his advice does seem to go somewhat against conventional methods and wisdom, notably I have read in numerous posts NOT to wipe off with a cloth before priming, as small fibers/particles can be trapped underneath.
Both tech support reps were very patient and helpful!

Thoughts????

 

Interesting that they only mention an Alodine application in areas that are going to be faired.
The Interlux website suggests (for aluminum) to clean it with their solvent to remove residuals from sanding, then apply one coat of 2000E before fairing.
Then up to 4 more coats to complete the prime coat.

Ensure that you read the data sheets as many thinners are unhealthy to breath.

Also the application procedure specs out a finished mil thickness. Your paint supplier can provide the credit card sized thickness tester that you use are you are applying the coats to ensure that you get the thickness that you need

 

Hi Barry. Just to clarify. The Interlux rep advised not to use anything under the I2000E other than denatured alcohol or another regular wipe down cleaner likely such as their 233 mentioned in the I2000E prep advice but NOT acetone. But he agreed with the West systems rep that I would want aluma prep or similar cleaner and then alodine on just the recessed areas to have epoxy over them.

 

I may call Interlux tech support again, but knowing that there will likely be other areas that could use slight fairing in addition to the most pronounced areas...therefore, rather than using West systems epoxy, with fillers for the worst areas - I may just use Interlux's fairing product, which is compatible for I2000E - for all of the fairing - it says it can be up to 3/4" thick....

It says to apply between coats of I2000E - I am not sure if fairing in between primer coats would grip quite as good as epoxy on the bare metal, but if it has acceptable strength, it would be a lot easier ...

 

Called Interlux tech support again, and revised plan a bit.
Instead of wiping down with a cloth and cleaner/ denatured alcohol after sanding, I will just wipe / dust off with a new broom and/ or old hair dryer perhaps....don't have a leaf blower.... and I don't want to use shop air as it would contain some moisture.
I do have a cheap air dryer / dessicant canister on my plasma cutter, which I could redirect to an air hose, but it may still have contaminates..

He also said I could use 40 grit rather than 80 grit - they normally speak to pontoon boat owners so hence the 80 grit recommendation, but my hull is 3/16" tick so 40 grit is fine as well.
Also, he clarified that first coat of I2000E has to be dry (about 24 - 36 hours) before applying the Watertight fairing compound to areas needing light fairing.

so new plan:

Approach B according to Interlux and West Systems tech support)

1. Sand worst recessed spots with 40 grit.
2. Wipe / blow off
3. use Alodine on recessed areas and then apply West systems 105 with fast hardener for low temps, and use either low density filler 407 or first apply more structural coat of codical sillica, keeping a bit low as not easily sandable and then follow with coat of low density filler for easier sanding.. Sand fair..
4. Sand entire hull bottom with 40 grit.
5. Wipe with new broom and/or blow dust off with old hairdryer.
6. Apply 1st coat of Interprotect 2000E within an hour of blowing dust off, thinned 15 - 20%
7. When 1st coat is dry (24 - 36 hours), apply Watertight 2 part fairing compound to any areas requiring fairing.
8. Sand above spots fair and Wipe off with cleaner.
9. apply 2nd coat of 2000E
10. While still tacky apply 1st coat of antifoul - Trilux 2
11. after 1st coat dries, apply 2nd coat of Trilux 2

 

I used an aluminum coating from progressive epoxy polymers in Pittsfield nh. Great quality stuff