Rectifying Spray Painting Faults
Nobody is perfect. Painting professionals will mess up much more rarely than DIY painters partly because they have more experience and practice, but mostly their facilities are set up to avoid common problems.
Most paint faults can be rectified reasonably well with time and effort and preparation.
Tiny blisters in the paint can sometimes be caused by overly thick application of paint allowing the outer skin to dry before the solvent has evaporated from underneath.
More often for DIY sprayers it's caused by water or oil contamination from the compressor. The effects are often not apparent until long after the paint process when the finished paint is heated up by the sun. Avoiding water contamination is covered in the air system page. Rectification would involve sanding below the level of the blisters and spraying further colour coats.
Here's a close up of a panel that has been painted on top of overspray. The overspray stands out like a sand paper finish and the only option here is to sand with 800 grit paper and spray another coat. It's a 2-pack problem. 2-pack takes a long time to dry, so overspray is still wet when it hits the next panel. With cellulose the overspray will normally be dry so it would wipe off a panel.
The overspray is on the top of an outer wing panel and came from the engine bay which had been painted a few minutes earlier. The outer wing panel had previously been guide coated and sanded, and the overspray could have been avoided had the engine bay been painted before wet sanding the outer wing panel. Another option would have been to use masking paper to protect the panel.
Overspray can be reduced by reducing air pressure to the spray gun (see the spray technique page). Overspray on top of paint is easier to deal with. On a finished panel it can be removed reasonably well with T-Cut.
Drips and sags
If you see a drip forming while painting you can reduce the damage by spraying air at it through the spray gun. The air can hold it in position and dry it out reducing the rectification work later. The finish will be terrible and it will still need rectification. Never try to wipe out the drip - always leave it to dry before any rectification or else you could take out previous layers of paint.
Here's a nasty drip. It's beside a nasty rolled edge where a fair bit of paint went on at a sharp angle. I nearly got away with it, but for the bowl shaped depression just visible at the top of the photo which must have collected paint aimed at the rolled edge.
To avoid the drip in the first place I could have painted the inside of the rolled edge then let the paint dry before painting the rest of the panel.
Drips can be sanded out effectively, but the drip is far thicker than the thickness of the rest of the paint on the panel, and normal sanding would wear through the surrounding paint long before the drip was anywhere close to level.
Wrap 400 grit sandpaper around a small wooden block to ensure the sand paper sands only the run and not the surrounding paint. An alternative is to use a nib file. This is a small file attached to a wooden block which can be used to remove the build up of the drip without sanding the surrounding area.
Once the level of the drip is close to the level of the surrounding paint a sand of the whole area with 400 grit, followed by flatting with 800 grit to remove the scratches will remove the drip altogether.
I've sanded into the primer around the location of the drip. The local area will need another coat of colour which will be flat sanded using 800 grit with the rest of the panel before applying a final colour coat.
Drips are caused by excessive application of paint, or in this case too many coats in the same area before the previous coats were dry. Planning the job better would have got around the problem for me.
It's better to end up with a little orange peel rather than drips. Orange peel could have been flatted without going back to the primer, and even a final coat could be wet sanded with 1200 grit and T-Cut for a smooth finish.
Fibres, Flies, and Dust
If there are small fibres on the panel the paint will be attracted to those fibres causing high spots very much like small drips. A lot of paint would need to be sanded before a smooth surface was available for the next coat.
Flies are worse trouble for the DIYer. Our ventilation tends to involve open doors rather than extractor fans, and the flies will come in. Keep an eye out for them and blow them from the panel with air from the spray gun before painting. Some colours are worse than others. Yellow is terrible for flies, and light colours are worse than dark colours. If they land in finished paint don't try to remove them while the paint is still wet. Let them dry into the paint and sand them off in the same way as fibres once the paint is dry.
Preparing the workshop by lining the walls, floor and roof with plastic masking sheet will reduce problems with dust. Again this is mostly a 2-pack problem. Cellulose will dry before the flies or dust spots the newly painted panel.
Painting over fibres left on a panel after wiping with a cloth is easily avoided by DIY painters. A tack cloth can be used to remove the fibres before paint. A tack cloth (or tack rag) is a cloth soaked in adhesive, and a light rub over the panel removes fibres. The particles in the photo are inside a door panel and were blown into the paint from the recesses of the door by the spray gun pressure. Wear gloves when working with panels - oil from bare hands can cause fish eyes (which are covered later on this page).
A first time sprayer doing things right will probably end up with a lot of orange peel. A lot of practice is needed to reduce orange peel without getting runs, though if your paint is orange peely and you never get runs you might need thinner paint or more air pressure (or more likely a better spray gun if you are running with one of the £10 jobs).
There's a balance between orange peel and drips. Professional painters will have had a load of practice and will take the balance far closer to the drips (without actually ending up with any drips) than we might manage. Their equipment helps too.
To rectify orange peel in primer a flat sanding with a guide coat is effective. The second to last colour coat could also be sanded with 800 grit, or if a flat finish was required the final colour coat could be sanded with 1200 grit sandpaper prior to polishing.
Fish eyes are caused by contamination on the panel (normally from silicone) that repels the paint as it is sprayed on leaving small holes. The best way to avoid it (apart from reducing contamination sources in the first place) is to paint all of the colour coats in a short period of time.
The painting environment should be completely clean to avoid contamination. Plastic sheets covering the walls, roof, and shelving in the workshop should get the DIYer most of the way there. During paint the workshop should not be used for anything else. Don't use any silicone products in there, oils or lubricants, or even other paints - hammerite contains silicone. Silicone is also found in some car polishes, most plastic polishes, deodorants, fibreglass mould release agent etc.
The panel to the left had nearly 2 weeks to gather contamination due to delays caused by bad weather. Had I painted the second top coat within a day of the first I suspect I would have avoided the problem.
If the problem occurs during painting use brake cleaner to clean the panel, use stopper to fill the fish eye marks, then clean with brake cleaner again before spraying. If that doesn't cure the problem it's possible to mix silicone additive such as Silistop with the paint. These products contain silicone so any further coat of paint on that vehicle would also require the additive, and the spray gun can become contaminated by silicone.
Fish eyes can be very frustrating. The suggestions above did not work for me. The source for silicone can be difficult to determine. I suspect the cause of my contamination was sanding dust from a glassfibre car. Either the contamination was still present or I was moving it around the panels rather than removing it. As the workshop will probably still be contaminated I would probably run into the same problems, so for the final coat I have hired a professional sprayer.
Blooming is a milky film that appears over the finished colour coat as it dries. It's due to water condensing on the paint surface as it dries in a cold or humid environment. A paint shop will normally be heated to 20C, but most DIY painters will avoid the problem simply by painting during the summer on a dry day.
To evaporate, the solvent takes heat energy from the coating and substrate, dropping the temperature on that surface below the Dew Point. If the Relative Humidity is close to the Dew Point already, moisture condenses out of the air onto the paint. Cellulose paints had fast evaporating solvents acetone and toluene, making them very prone to bloom.
Blooming tends to start to be a problem below 10 degrees C and can be avoided to some extend by anti-blooming thinners which are slower drying than conventional thinners.
2 Pack vs Cellulose
2 pack is more prone to just about every paint fault than cellulose. It takes much longer to dry, so overspray will stick to panels, flies and dust have more time to settle on the panels, and it has more time to drip. The only paint fault that is worse with cellulose paint is orange peel. Cellulose doesn't flatten after spraying to the same extent as 2 pack so generally needs to be polished or sanded after spraying to remove orange peel.