MG-Cars.info

Welcome to our Site for MG, Triumph and Austin-Healey Car Information.

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG Midget and Sprite Technical - What am I doing wrong with my paint?

I am trying to rectify some blemishes on the bonnet but the more attempts I make the worse it gets. I resprayed the car in cellulose 2 years ago and most of it is excellent, but I got some primer cracking on the bonnet and I'm having another go at fixing that. I have even taken it down to the metal, etch primed, and done all the usual preparation. Surrounding area flatted with 1500 grit, primer flatted with 800, cleaned with panel wipe. Colour coat air brushed. But despite aggressive flatting I am unable to blend the patch completely. Some areas do, others don't. The darker line in the photo is the boundary between new and old paint. The more I flat and polish, the darker it gets.

Before you ask, yes the paint is all the same batch. It's a week since the colour went on. I am using fast colour thinner. I thought I was polishing through to the primer, but no I'm not - I can see that the darker band overlays the previous paint.

I have done lots of similar patch repairs over decades, with no problems, so what is going on now?


Les Rose

Les
I don't have an answer but I'm getting a similar problem with BRG celly from Jawels when patch repairing/ improving. It only really shows up close so I've virtually accepted it now. Everyone will tell us to use 2 pack I suppose but that isn't the right answer is it. My bodyshell was sprayed with etch primer, like yours, and then high build celly primer etc.

I have had problems with the colour coat sinking into filler patches but that is probably due to me rushing towards the end of a "weather window" and not flatting/ guide coating properly.

Sorry, I'm not really helping!
Bill Bretherton

Bill, I think the paint formulation has changed a lot since my callow youth. It is nowhere near as forgiving. Back in the day I had no idea what I was doing and my cars always looked wonderful.
Les Rose

I don't know, but darker patches sounds like the colour pigment is migrating (flowing) within the solvent before it evaporates. Maybe try lighter coats, or pass the gun across the panel with a bit more delay at the end of each pass? So it dries faster before the pigment flows?
GuyW

Les,

Did you mix it really really well if it has been standing for a while. I've seen it before where some pigments seem to settle out more than others and it can be really difficult to get them back into suspension once settled to the bottom of the can. I haven't got any cans of any definite colour that need to be kept blended but have a tin of high zinc paint which I invert most times I pass it to help keep the zinc suspended after leaving it for an extended period and having it settle out and even after a lot of stirring I could still feel some on the bottom. I do have a dozen or more tins of mainly celly in named shades but I keep it for coating things where I want a quick drying coat and am not worried about colour, it came from a local vehicle place when HSE found they had far more on the premises than they were allowed to keep, some so old it was Dockers paint in little milk churns, I wish I'd kept one for myself.
David Billington

I'm thinking that maybe you're using too much thinners trying to get the paint too perfect coming out of the gun and not enough drying time between coats, and maybe too much paint per coat and the thinners is soaking in too far before it goes off
You could try with less thinners and higher pressure and stay further away from the job and just dust a coat on then plenty of drying time between coats, don't try and get it all covered in one go, take your time and light coats----plenty of drying time between coats,
Haven't really had this issue with red but yellow is a real terror for it, you need the area nice and warm and lots of drying time--if you rush yellow the underneath comes up and looks at you every time
willy
William Revit

Thanks chaps. It's not a matter of paint not fully mixed, as the dark area is the same coat as the lighter area adjacent. Something odd happens at the edge of the new patch. I do give light coats and at least 20 min between. I will have one more go before the next show on 18th. I'll try less thinner and higher pressure.
Les Rose

Les, that's why I mentioned pigments 'migrating' before the thinners flashes off. From an initially well mixed paint in the gun, the pigment then moves and concentrates in bands, giving the shade variation.Its a bit like a paint run, but just of the colour pigments within the paint film. Solution is thinner mist coats with time to flash off before the next pass or coat.
GuyW

Guy, if you don't mind can I do a little adjustment to what you have written--

"Solution is thinner mist coats with time to flash off before the next pass or coat."

I've written the same thing myself and thought maybe that could be confused with---more thinners

maybe---
---Solution is lighter mist coats with time to---------

just dust it on
cheers
willy
William Revit

Yep, Willy, 'lighter' coats is a better way of saying it. I did not mean more thinners! The other thing is to allow more flash - off time. It's easy to hurry it too much especially when spraying individual panels rather than working around a whole car.
GuyW

Lighter coats makes sense but they tend to dry as soon as they touch the metal surface, especially on a warm day i.e. the paint doesn't "wet" the surface. You then get a rough finish which needs cutting back a lot and you need many coats to get enough depth. It's then very easy (I've found) to end up with a too thin layer towards the edge of a panel, say, so you then risk cutting through almost to primer or you get a dull area that won't shine up as much. I'm sure it's all down to technique and decent kit but it's difficult to get it right on a one-off job.
Bill Bretherton

From my experience with celly this current warm weather is slow thinners conditions not fast as Les stated he used.
David Billington

Yes I do get dry edges that are hard to flat down. I'm confused now. The weather is not really warm, it's 20ºC in the garage. Do I want the solvent to flash off quickly, so fast thinner, or slowly? To get thinner coats I need thinner paint not thicker.
Les Rose

It doesn't have to flash off fast just ordinary mp thinners and again not thinner coats--lighter coats---just bump your pressure up a few lbs. and step back off the job a bit and just mist some paint on---not so it looks wet ,just a mist and then give it heaps of time to dry off then another light coat-----------------
William Revit

This thread was discussed between 06/06/2023 and 11/06/2023

MG Midget and Sprite Technical index

This thread is from the archives. Join the live MG Midget and Sprite Technical BBS now