painting process seams

[Bradley D]

We occasionally make larger signs out of sheet aluminum that have seams. We have struggled with hiding the seams after applying bondo and sanding several times. The finish hasn't been as good as I would like. Can someone suggest a good method or resource on getting a good finish. I'm trying to order MP 6002 polyester high build primer to see if that works better than the 2.1 epoxy primer I currently use.

 

[visual800]

its its REAL aluminum and not maxmetal we always lap the panels and let it go. If it maxmetal it gets butted together and thas it, it is what it is

 

[Bradley D]

My process is taking .090 aluminum butted and there is a back piece of .090 going along seam with MMA adhesive. Seam is rock solid, I'm just looking for the best method to get a perfect finish without seeing either the seam or the bondo lines. My application requires a high level of finish. I suppose it is mostly the prep work that I am struggling with. I usually sand with 180 grit sand paper on wood block and finish off with a maroon 3m scotchbrite pad. I don't sand after putting on the primer so I want to try a high build primer with sanding but not sure if there is something else I could do to get a flatter surface for a good finish (no visible lines from bondo or seam observed with close inspection)

 

[JBurton]

For one, you may want to bevel the edges where they butt together, to give your bondo/body filler some more surface area to adhere to. Bondo spot glaze putty is what we used to always use, but I have a new painter who has been using the ASI 55420 construction adhesive to fill gaps. From there I think it's just a nice sand job. Run down to the HD and grab a husky orbital sander and pick up some hook and loop pads from Reece, and order a yellow 3m hookit sanding pad to replace the husky one. Those yellow pads have more give, and prevent gouging, while giving even pressure.
I'll ask my painter later today about it, he just filled the edges of a panel glued to 2" tube, and I can't differentiate the panel edge from the tube. While not as impressive as a hidden butt seam, he's got some chops for certain.

But, I'm a big fan of working with oversized aluminum to prevent seams, or letting them go without a second thought. I realize you're selling a premium letter, so getting a great background is pretty crucial, but at the same time, whose standards are you trying to satisfy, the customer's or your own?

 

[Bradley D]

I am not extremely picky but my clients are and my router is only a 4x8 which is fine since I mostly do smaller channel letters but when I get a 5' circular logo it kind of sucks. I have tried nearly everything mentioned that you do with some success but it seems that whether I spend 4 hours or 15 minutes I'm still left with a seam that is somewhat noticeable. I've been called out on it once so I'm trying to remedy the problem. Thanks for your input and if there is any trick to getting a better finish I'd love to know it.

 

[JBurton]

One thing I'm wondering, are you taking the mill edge off your sheets? To get the flattest panel out of a sheet of thicker metal, like .090", you have to take off the outer .5". Aside from the edges being dinged or marred in shipping, this area has a deformation from whatever monstorous contraption that rolls the stock flat. But surely you've been seaming smaller stuff that was definitely cut down well from that edge.
Picking on my painter for a minute, he's using 220 to finish, and basically over gluing panels, so the squish out becomes the seam filler. That and he likes the satin finish because it's way more forgiving than the car clear coats that he's used to.

 

[Bradley D]

Not sure what you mean by outer .5" is that just outer edge of sheet?

 

[JBurton]

Yessir, if you get a fresh shiny sheet it, unpainted, lay it on a flat surface, and look across it, you can see a slight deformation toward the long sides. The short sides are sheared, and have their own different deformation, but this one is visible in the flat plane of the sheet.
I caught my painter for a few more minutes, and asked him about just doing a full on perfect butt seam. He said he'd coat the whole area in body filler, then sand it down with a block slowly. He also said you can do a lot with a high build primer, multiple coats, and doing a light sand between coats.
Where is signbrad? He's going to be the best source of advice here. He'll probably also fix a couple of my statements. This is the one area I'd consider myself unfamiliar with, compared to most stuff.

 

[Bradley D]

Awesome. very helpful.

 

[bowtievega]

JBurton has some good info there. It also depends on whether that finished panel is rigid or not. Does it have a frame behind it to keep it from bending when you have to move it around? It can be easy to crack that filler in your seam if the panel has too much bend to it. The thinner the material the harder it will be to get it truly smooth and flat as the material starts to have some waviness to it. Block sanding like you are doing is the best way to flatten filler as long as you are careful to keep and cross-hatch pattern and alternate sanding directions so you don't create slight indentations. You can rough it down with D/A sander then finish it by hand. We used to use a product called Marhyde, its a high build automotive primer. Sands super nice, great for applying over bodywork. I think it has been phased out due to EPA regs so we had to swtich to something else. I will as my painter when he comes in about what we have in shop now. Its a good idea to epoxy prime over you bodywork to seal it before painting spraying another primer. Bondo can be pretty porous and suck in that paint unless you seal it. We do mostly satin finishes as well and that also helps with not noticing small blemishes as you don't get that mirror finish to reflect off of your issues.

 

[Bradley D]

I usually make stuff like this out of 5052 H3 aluminum 1/8" with MMA adhesive backer. I guess what I really need to try is a fast dry high build primer. My epoxy primer says you need to wait 4 days in order to sand it. I don't wait 4 days but I also haven't done much sanding beside getting the bondo flat. I think the different absorption levels between bondo to aluminum is my problem. Thanks for your input.

 

[JBurton]

You can rough it down with D/A sander then finish it by hand

This surprised me, I was always under the impression that an orbital provided the best finish, but my painter was quick to tell me that finishing a butt seam by hand with a block was the way to go.

On the edges of sheets, it's really smaller than I was imagining, though I know I've gotten some sheets that were worse than the one I got a picture of, this one is only about an 1/8" of 'non flat' area. This is a sheet of .125". The right hand side is sheared, the top is the milled edge, and you can see the difference in finish running along the top edge. It's pretty flat, but there is a definite deformation.

 

[Bradley D]

Thanks for the picture. I often have issues with my channel letter coil as well having an edge that has slitter issues. I'm probably the only one that notices if the burr is beyond spec for such things because I do trimless letters. Anyway. I was going to try Evercoat featherfill g2 primer to see if that works. I am doing a sign that is 14" x 4" with one seam.