MG MGB and MGB GT Tech Talk
I've gotten the underside of my car media blasted and I painted it with POR-15 (or chassis saver - I can't remember - but they are pretty much the same thing).
I'd now like to apply truck bed liner. My question is - do I need to put a self-etching primer on first or can I apply right over the POR-15?
The POR-15 is for corrosion protection - the bed liner is for road noise.
Nothing's going to stick to POR15 unless it's properly primed. The company sells Tie Coat to use for if you want to paint over the POR15. I wouldn't chance using anything other than their product if you want to be sure it will last. I've used their Tie coat over the POR15 and then sprayed paint over it for wheel wells and it has held up perfectly for quite a number of years.
Emil nailed it but I would like to add that for road noise, you may be better off with insulation padding than undercoat. I suspect that it would not only hold up better but insulate against heat and vibration better than undercoating.
I find that unless you paint over rust, POR peels off. Painting POR without the POR supplied primer also will not work.
My personal belief is that noise and heat need to be defeated BEFORE they are inside the car. In a proper istallation, the foil backed daper/heat reflector will be on the outside of the car.The heat would be reflected back at the source and the noise would be damped before it reached the tin to even start its journey throughout the cabin. So, if you can get reflective bedliner, by all means spray that suff on thick under the car!
However, for purposes of damping noise, a rubberised coating is going to be more effective than the higher durometer bedliner.
Pete
Pete, I agree with Gary (gasp! ;) ) that POR is quite permanent if used on a sand blasted surface. I've personally never used primer before applying POR and have never had it peel. I would agree with you 100% if we're talking about smooth metal however, since POR appreciates a textured finish.
Also, do you not find problems with trapping water between insulation and the under body areas? I fully agree that insulating outside is the best but rust would be a concern to me. Any personal experience you can share in that regard?
If you plan to spray with rattle can bedliner check the instructions supplied. All that I have seen on the market require scuffing or sanding the surface that it will be applied to. That would not be a fun job underneath the car especially coated with POR.
I never had a problem with POR15 even when applied to new metal as long as the surface is properly prepared. It will not stick to a smooth surface - it has to have some tooth. You can either abrade it (sandblasting works great) or treat it with a metal etching solution. If it peels, chips or flakes either the surface was too smooth or it was contaminated.
Notice I said sand blasting because other Medea blasting such as soda will not rough up the surface.
Should have said sandblasting, because that's what I had done. E-mailed the POR-15 guys, and they said to go over the entire surface with 320 sandpaper. Of course on the underside of the car, there isn't really a way to do that efficiently using a power sander, so it is all handsanding. UGH! I'm not sure I'm that invested in the bedliner idea. Maybe I'll just go with foil sound insulation inside the cockpit.
I sanded a lot of the areas on the Pantera which were POR 15 coated and that worked well for me.
You don't need to sand POR15 if you use their Tie Coat. It's specifically designed so you don't need to sand it first. That's how I've always used it. I don't know who at the company told you to sand it first, but I think their just being overly cautious about giving recommendations to such a specific question. Of course it won't hurt to sand first, but I would then still use the Tie Coat product after that.
Just for info, sanding is recommended for top-coating POR15 with more POR15. That may be where the confusion lies.
This is what the POR-15 rep said, "Hello Bob. If the POR-15 has completely dried, then you are going to have to sand the POR-15 with 320 grit sandpaper. Blow off the loose dust and then you are ready to go."
Here's what I said, "I was afraid you were going to say that! That's what I get for buying such a tough product!"
I've done this.
Gas tank exterior POR-15, light sanding, brushed on bedliner. Worked great!
Rusty floorplans: POR-15, forgot to sand (or use tie coat), brushed on bedliner. Bedliner came off in big sheets! But the shiny POR15 still looks great!
Tim
POR15 reads not for direct exposure to sun light. I guess it doesnt like UV.
With NEW metal, POR must either be treated with copious amounts of Metal ready to get a good etch on the surface or a combo of sand blasting, and metal ready, which works best. It WILL NOT peel off if you do that.
As for the Bedliner. if its for noise, a better suggestion would be to use something on the inside. Like the Foil backed heat/noise inhibitors that go on the inside of the cockpit.
PLUS, just the added POR15 itself is going to greatly reduce road noise. If you were to drop a bolt onto the bare metal floorpan, you get a noise DINNNNGGGGG!
When POR15 is applied, you get a THUD! Try it!
Ya, applying the Bedliner onto the bottom of the car without sanding EVERY NOOK AND CRANNY is going to cause more harm than good. BUT, the POR15 etch primers will work.
Use those, then paint body color.
B
POR15 is UV sensitive and will discolor with prolonged exposure. This does not harm the coating, it just doesn't look as pretty. If your parts are to be exposed to direct sunlight then use one of their UV blocking top coats.