Car primers and guide coat flatting

Self etch primer

For outer panels from bare metal I'll always use self etch primer. It's recommended for bare steel, aluminium or GRP, and it won't flake off like a filler primer can. It contains an acid which burrows it's way into whatever it's being sprayed onto. That's handy for bare metal with body filler, but it can burrow under and lift existing paint or sealant, so it's not so good for repairs.

Paint tends to run when applied directly to bare metal (even more so when bare metal is mixed with a plentiful quantity of body filler - runs start at the interface between the filler and paint). The way around this is to apply a thin mist coat of primer before the first complete coat. Here the panel has been sprayed with a mist coat which will be allowed to almost dry before the full coat goes on.

After the mist coat the full coat should go on as normal without runs. The photo shows a panel mounted horizontally, but the mist coat helps even more with vertical panels.

It's well worth using a high build self etch primer rather than the thin versions. It will generally be a different colour to later coats of high build primer and allows a safety margin for rubbing down. If the self etch primer colour is exposed while high build primer is being rubbed down it's a sure sign that the panel has been rubbed down far enough and rubbing any further will expose bare metal.

High build primer

A high build primer is the next step after self etch priming. The high build primer adds a relatively thick coating of paint which is sandable and is used to remove the imperfections in the panel. Normally two coats of high build primer will be sufficient.

High build primer is available in cellulose and 2-pack. 2-pack requires a greater level of safety equipment than cellulose, but will tend to go on thicker and smoother than cellulose, and does not suffer from sinkage to the same degree. I've applied two coats on the outside of the car and a single coat on the inside.

Care should be taken not to build up excessive paint thickness at door edges or they will be prone to chips.

Flatting with a guide coat

A guide coat is a thin mist of colour applied over high build primer and then wet flatted off. Initial sanding will remove the high points, and it is only when the whole of the guide coat has been removed that the high points of the panel will be level with the low points - then the surface will be completely smooth and ready for top coating.

Guide coat sanding is done immediately before the colour coat. My approach for a guide coat is to mix evenly colour with primer then add at least 75% thinners. The primer content aids sanding and thickens the guide coat so it doesn't disappear before the panel is flat. Using the top coat colour prevents problems with painting over any guide coat that isn't completely sanded off.

600 grit wet sandpaper is appropriate as a base for the colour coats.

I'm sanding with a sanding block just about visible on the door panel above the part I'm flatting. A sanding block is useful as it doesn't follow local depressions in the panel as much as hand sanding.

The direction of sanding should be changed frequently when block sanding. First along the panel, then up the panel, then 45 degrees, then 45 degrees the other way.

This is a flat panel. For a panel with a sharp single curve I might use the block a little first to reveal the low spots then finish off with hand sanding. For a sharply compound curved panel I'll forget the block and go with hand sanding.

The areas still coated with guide coat after further sanding are the ones I got close but not quite right with body filler.

It's very useful to have a first high build primer coat in a different colour so you know when you have sanded through the body filler and are getting close to exposing bare metal. I've used high build self etching primer on this car, so stop when I see the layer of yellow paint.

You have to be careful on panel edges though - a single wipe with the sandpaper can remove the filler primer and all of the self etch in one movement. If this does happen spray a little more primer on the area and flat it into the panel without guide coat. Edges are best finished by hand sanding.

Don't use any wetting agent (such as soap) during wet sanding - it can be difficult to completely remove from the panel and can contaminate later coats, and in any case it's not necessary for primer.

Here's the finished panel. The photo doesn't show up all the bits I've left. The area around the door handle will be hidden by the handle itself, so there's no point risking sanding too much around those sharp edges. Also the window frame on this car has a lip left when the panel was pressed which could easily be sanded to bare metal, so I backed off on sanding before the guide coat had been removed.

Flatting with a guide coat leaves a good flat surface to spray colour coats onto. Any paint disasters from here should be sortable by sanding or polishing colour coat without any risk of going back into the primer, and because the base is flat it will be possible to flat the colour coats to leave a completely flat mirror finish.