se_racing

hello all. this is my first time posting here and im hoping someone can give me an answer to my question. i have a 24" SE Racing Floval Flyer racing cruiser with a aluminum frame that id like to paint.

most forums ive been to talk about stripping the frame completely of its old paint. then they mention how aluminum is not a very porous metal so i would need to apply a chemical to the bare aluminum to open its pores so that the primer has something to "bite" into.

my question is this..instead of stripping and then treating the aluminum with that chemical...couldn't i just lighty sand the surface of the existing paint to get a slight texture that it would make the paint "porous" enough for the primer to hold? then apply the primer directly on to the sanded paint. followed by two or three coats of paint. and then a layer of clearcoat. i just want an option to the harsh chemical method. would my idea work well? would the paint hold up well? any ideas or suggestions are welcome and appreciated. thanks.

Miguel.

DannoXYZ 

Problem with adding paint on top of existing layer is you'll end up with a lumpy finish. It's hard to sand the entire frame perfectly even without going down to the metal. But yeah, it'll work if you just rough up the original finish.

Another problem is that a really thick coat of paint with 6-8 layers will chip easier than the original 3-layer paint.

The trick is to use an aluminium-specific primer, like zinc-chromate.

vpiuva

If you're just going the rattle-can route, you might not want to bother with stripping as it will chip anyway. If you're going to powdercoat or use a 2-stage type paint, unless you're matching the old paint it will definitely be worth your while to strip entirely. Some BF members have been lucky enough to find media blasters that have stripped frames for $25.

Retro Grouch 

I haven't found that to be true.

I've painted several frames with rattle cans. I don't claim to be super knowledgable and I don't do anything special. I use a chemical stripper to remove all of the old paint. I spend a lot of time with primer and sandpaper. I give the primer about a week before I shoot the top coat, I don't clear coat, and I wait at least a couple of weeks before assembling the bike.

The bike that I ride most often has about 3 years on this paint job and doesn't have any chips in the paint.

oilman_15106

Just curious, since this is the first non chip rattle can paint job I have ever read about. What kind of paint are you using?

Retro Grouch 

I don't even know. Whatever they had at the hardware store. Probably some Rust-Olium product. I take a good amount of time with surface preperation, I give the primer a good amount of time to cure, and I don't go overboard with the paint - about 3 thin coats. Like I said, I don't do anything special but I'm careful about how I do it.

Nessism

In the engineering world paint durability is gauged based on tests like corrosion durability (measured in a salt spray cabinet), chip resistance (gravelometer test), adhesion test (cross-hatch and tape method), etc.

On a relative scale for durability considering the various paints available, rattle can paint falls at the bottom of the list. It will hold up fairly well if you keep it out of the elements and don't touch it, but if you subject your bike to rough usage such as riding on gravel roads, the paint will chip/scratch up pretty bad.

Regarding the OP's question, I'd say to wet sand the original finish with 400 paper and spray on top of that. It won't look pristine, but will be serviceable enough for a rattle can job.

Retro Grouch 

So how about a recap?

You have one poster who said that painting on top of the existing finish will make it chip more easily.
You have two posters who say to scuff and paint over the old finish but that it's going to chip easily.
You have one guy who says he strips off all of the old finish and his paint jobs don't chip.

What do you think?

Nessism

Opinions are unreliable, even mine. Go to the "manual" so to speak, or the paint manufacturers instructions in this case. Just about all paint people will tell you that it's okay to scuff the old finish and paint on top. If the film build is high though, it's best to take it down some or remove it all together if chip durability is top priority. Problem is that you need a special tool to measure paint thickness. Considering we are talking about a rattle can job here, I say just scuff and spray - no reason to split hairs about durability when you are using the least durable paint available.

HillRider

First, I assume it's a welded Al frame, not bonded. If it's bonded, like the late '80 to mid-90's Treks, all bets are off.

If you decide to strip the old paint be careful of the type of stripper you use. Do not use caustic based strippers as they will attack the aluminum. Use a methylene chloride or similar solvent-based stripper and plan on lots of elbow grease and cheese cloth rubbing to get the paint completely off and out of all of the crevasses. It's going to be a lot of work.

Retro Grouch 

The last one that I did (summer 2006) was a Raleigh Technium, bonded aluminum mainframe with steel forks and stays. I did use methlene chloride to strip the old paint - messy and took about an afternoon. It's painted with Rust Oleum satin black. I built it into a fixed gear so I don't ride it very much but virtually all of it's miles are on the crushed limestone Katy Trail. Actually it has 1 tiny nick from the water bottle but none under the down tube or on the chain stays.

HillRider

I was concerned the solvent would also attack the glue used to bond the frame joints. I guess you diddn't have any problems with that but it is a consideration.

Retro Grouch 

To be honest, it didn't blip my radar at the time. I hadn't considered that some paint strippers might damage the aluminum either. I figured the frame had essentially zero value so I didn't have anything, other than my time and materials, to lose. Live and learn. Fortunately for me, everything seems to have worked out OK.

Joshua A.C. New

Methylene chloride warns of spontaneous combustion on contact with aluminum.

So, naturally, I ground up some aluminum powder and put it in some methylene chloride outside the shop to see what would happen. My dad and I stared at it eagerly for a few moments, then poked it with a stick, then swirled it around, then felt for heat...

Nothing.

I suspect that the real concerns are when the aluminum is in huge quantities and the ambient temperature is high. We sure couldn't get it to do anything fun. It just made that horrible, nasal hair-curling smell that it makes straight out of the can.

The stuff doesn't go through everything. I could only get it to take off the top layer of paint the last time I tried it; it wouldn't attack the primer at all. And while it removes adhesives, the stickers themselves sometimes (and only sometimes — it clearly depends on the material of the stickers) form a resist, which means they slowly peel up from the edges. You can get more nassty chemicalses under there, which will continue to attack the adhesive, but it's kind of tedious. Once you can rip it off, do; then the remaining adhesive will all turn to goo and flake off with the paint.