Topic: Help painting plastic tail on 750F

[dlhoulton]

The subject is the plastic tail section on my 77 CB750F. I hand sanded all the paint off down to bare plastic. Did get a little over aggressive in one area seemed to have broken through to what I can only describe as a very thin skin to plastic on the piece. No matter what grit of sandpaper I use the edge will not feather. So I decided to go ahead and prime it and fill with build primer. I first primed with Duplicolor self etching primer and all went well. You could see the rough circular area that had the edges. After a full 24 hours of drying I then applied some Duplicolor high build sanding primer. Set it aside to dry. After another 24 hours I looked at the piece and the rough circular area edges all bubbled up or what I would call blistered. Only the rough circle edges, not the center area. Sure could use some advise on what I'm doing wrong. Should I not be using the self etching primer and/or high build sanding primer on plastic pieces? I'm prepping all parts: tank, side covers and tail piece to paint with Eastwdood 2K paint and clear coat.

[CB750R]

Using an etching primer on a plastic substrate is generally not reccomeded.

I suck at painting tried several times to rattles can my bike never had issues with the primer stages. I would sand off all the etching primer use a good wax/ grease remover, and just use a high fill primer.

Wet sand with 400 grit the layers of high fill primer until your spot is gone.  This may take several paint and sand sessions.

[Terry in Australia]

It's probably a chemical reaction between the substrate and the thinner in the primer that you're using. You can buy a plastic primer that will put a layer of non-reactive film between bare plastic and paint and seal one chemical from the other.

You can also buy a sealer/primer to spray over an old paint job when you aren't sure what it's been painted with previously, but you don't want to take it back to bare. If you use one of these products, you can't sand it smooth after you've applied it, in case you cut through the medium and have the same problems again. Cheers, Terry.

[dlhoulton]

I've sanded off all primers back down to bare plastic. It seemed the Duplicolor self etching primer didn't cause any problems. At that stage it was looking good. Then when I used the Duplicolor high build sanding primer it caused the problem. I was trying to build up the area to be level with the rest and the center filled and excepted the primer, just the edges of the low areas. Seemed like a chemical reaction that bubbled up the edges/borders of the low spots. I will go look for a "Plastic primer".

[dlhoulton]

Calj737, that is butter smooth. Hoping for that look in my end results. This is what is happening just using Duplicolor high build sanding primer:


You can see the reaction is only on the edge of the area where I sanded a little over aggressively.

[nvr2old]

You did say that you got the whole part down to bare plastic, but it looks like where it's lifting is a paint line.  If so, your primer is reacting to that, because it's only lifting on the edge. Those rings still look like layers of paint to me.  I would sand it all down to a consistent bare plastic..(there are no layers in the plastic)..it'll be all one color, and start the high-fill primer session again.  Etching primers are for metal parts.

Good luck. 

[Terry in Australia]

Yep, I'd use some thinner to remove all that stuff and take it back to bare plastic, then use some proper plastic primer to seal it before you waste any more product on it. There are plastics, and then there are plastics, so no guarantee that Cal's method (which looks great, BTW) will work on your plastic. Cheers, Terry.

[dlhoulton]

I'm pretty sure it's down to bare plastic. This is doing just like my plastic inner finder did. There was no paint on it, but as I sanded on it I broke through a layer that reacted just like current problem. Only with the inner fender I was spraying Valspar spray paint for plastic (Lowes product). Had bad reaction. Sanded it all off and used Krylon fusion paint and it did o.k. With this piece the PO had painted it red on top of OEM paint which was the burgundy color.   

[jmac851]

Even though it says that 24 hours is sufficient, I've found that with the high build primer you should wait at least 48 hours before recoating. Sometimes more time is needed (3 or 4 days) based on the temperature and humidity. Be patient and let it cure.

[dlhoulton]

Just went out and tried some paint thinner to see if anything would happen. Nothing happened. I'm sure I'm down to bare plastic. It's like on the "show" side of this just like the inner finder it was shiner. And when I sanded to remove scratches and blemishes I broke through a layer that just wont feather. When I sand it, it definitely is a plastic layer. I'm using 120 grit for sanding so I just tried using Scotch brite pad and it seemed to feather/blend better. But I definitely removed a very thin layer of plastic. I say this because the sanding residue is black and does not react to paint thinner but does to acetone.

[mick750F]

It's probably a chemical reaction between the substrate and the thinner in the primer that you're using. You can buy a plastic primer that will put a layer of non-reactive film between bare plastic and paint and seal one chemical from the other.

You can also buy a sealer/primer to spray over an old paint job when you aren't sure what it's been painted with previously, but you don't want to take it back to bare. If you use one of these products, you can't sand it smooth after you've applied it, in case you cut through the medium and have the same problems again. Cheers, Terry.

   Though Terry and I usually seem to agree on beer, bikes and babes I'm going to go out on a limb here and agree with him on it being a chemical reaction. That was my first thought after reading the OP. After reading the rest of the posts I'm still thinking that there's contamination somewhere. Plenty of people over the years have done painting on the plastic parts of these bikes without any problems. If you don't know the the history of the bike/part then you don't know what some PO might have done to it. Okay I've just read your last post and now I'm thinking that you've gone deeper into the plastic that most have done. Could be time for bondo and feathering or if you've managed to go too deep into it to get good results...a new tail section.

   Now that I've reread your OP I see that you were very aggressive in removing the original finishes. Usually you don't have to go that far in order to refinish. You've gone deeper than most. At this point you are now a pioneer...let us know how you resolve this.

Mike

[dlhoulton]

Let me start by giving all those who gave advice an apology. I had reported here that I had removed PO paint and had sanded down to what I thought was bare plastic. I had thought I got over aggressive in my sanding but that was not the case. The PO had painted a very sloppy/runny and what I thought was spray can paint. It may have been but I suspect it was an automotive paint. Any way I keep noticing a "burgundy" type halo in spots. So I said what the hell and sanded a little more and finally got down to the OEM paint!!! The PO had applied some type of a black rubberized primer. As I was sanding it keep pealing (like when using an eraser) until it all came off and I could see OEM color/paint. Due to the fact that across the back of the cowl/seat piece there were some scratches I went on and lightly sanded off the OEM paint to what I know now is bare plastic. Applied some "adhesive promoter" and then primed with some "high build primer" for plastic.
I am at peace now!!!!!  Thanks for all your patients and advice.
As they say, "Never happened without pics":

[Terry in Australia]

Looks great mate, well done. Cheers, Terry.

[Don R]

it's called a repair ring