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Refinishing — Part 2: Application

Contents

Preparation

The amount of preparation varies depending upon the type of work being undertaken. If an undamaged panel is to be resprayed, the surface should be degreased, roughened with P280 dry or P320 wet sandpaper then degreased again. A similar procedure is followed for bare metal but using P120 dry sandpaper followed by a coat of etch primer which contains acids to form a good chemical bond with a metal substrate.

Filler

If a panel is damaged, more skilled work is required. An example of quite involved work is given in the Roof Rack Removal page where a combination of panel-beating, welding and filling are used. I am not an expert in sheet metal repairs and fortunately have not had to do much panel-beating which involves specialist tools to stretch and shrink the metal as required. Nonetheless, there is no substitute for beating a panel as flat as possible before applying filler. Filler is designed to be applied in thin skims of no more than a maximum of 3 mm. If there are 'hills' in the surface then these should be hammered flat as a sander will simply cut through all the filler in the centre of the hill.

Modern polyester fillers are mixed with a small amount of hardener that speeds up the curing from a few hours to around 12-15 minutes at room temperature. Filler should be mixed on a smooth surface; paint suppliers can sell boards with removable papers but I find a large glazed floor tile cleaned with thinners is just as good. Avoid mixing on an uneven surface such as corregated cardboard as it can introduce air bubbles and spoil the finish.

If the surface metal is relatively flat, a truly flat surface can be achieved with two coats of filler if applied and sanded correctly. I like to apply a crude skim of filler over the affected area plus ~5 cm (2 in). Filler tins come with a plastic spreader but I prefer to use wide reusable stainless steel spreaders from any paint supplier. Don't keep stroking over the filler to try to get it flat as it quickly tears after a few minutes of curing — the aim is to achieve a general build of filler over the entire surface.

Once cured, the filler needs sanding. At first I fell into the trap of using a fine machine sandpaper (P120-P240) which would 'polish' the hills rather than remove them. Now I cut 'through' the hills with P60-80 paper to achieve a rough but flat surface before applying a thin second skim, flattening with P120 followed by P240 prior to priming. This can be done effectively by hand but I use a good random-orbital sander which achieves a flatter surface more quickly. Using coarse sandpaper it is all-too-easy to cut right through the filler altogether. If this happens, stop immediately, apply another coat and if necessary hammer down any high spots the filler reveals. It is a good sign if filler is only being visually removed from the edges.

How do you know when to stop sanding? Continually feel the surface during sanding with the palm of your hand (not the tips of your fingers) and run your hand up and down. Sanded filler dust acts as a good lubricant and helps to feel any high or low spots. Do this throughout the sanding process. When the surface feels flat, ensure that the edges of the filler fade out (or 'feather') so that they look transparent. I have sometimes achieved a flat filler surface but then discovered a 'step' at the edges after applying paint.

Learning to apply and sand filler is a slow process. I would sometimes need four or five coats at first but I eventually learned to anticipate problems before they actually happened. But do remember: filler is no substitute to good metalworking processes.

Masking

Masking off panels that do not require painting can be a slow and fiddly task but it is worth doing properly as overspray onto good paint can spoil a job. Paint-proof paper can be purchased from paint suppliers but newspaper can be used providing at least two layers are used as it can become saturated and leak or even tear. Avoid buying cheap masking tape as it can leave behind a residue that is hard to remove or even lose it adhesion when wet.

Application

The prepared surface should be cleaned with an appropriate degreaser applied to a lint-free cloth or with pre-soaked panel wipes. Immedately prior to painting, a tack cloth should be lightly wiped over the surface to remove traces of dust. I cannot stress how important this is; even if the panel looks clean there are often invisible particles that can ruin a coat of paint so it should always be done as a matter of course. The workspace should be as dust-free as possible, with even lighting and maintained around 20-22°C (68-72°F).

Gun Setup and Control

The spray gun setup is described in the manufacturers' datasheets, including pressure, flow rate and distance. Primers require the greatest paint flow, basecoat at about 1/4 the primer flow, and clearcoat at about twice basecoat flow. Too low a paint flow will result in a rough, even dusty surface and too much paint flow will cause runs which are particularly apparent in metallic/pearlescent basecoats.

When applying, keep the gun at a the required distance and perpendicular to the surface at all times, painting right to the edge of the panel and over the masking to ensure an even finish. On edges with a large bend radius, paint the less visible parts first to avoid overspray onto larger areas. Cast the gun over the surface at a constant speed (determined with practice) with a 50% overlap between strokes.

After each layer, dispose of excess paint in an airtight container (NOT down a drain), clean the paint cup with universal cellulose thinner, then spray thinner through the gun until all paint deposits are removed. I haven't yet discovered how to dispose of unwanted paint; I have a large barrel that will take a long time to fill but I hope I can pay a bodyshop a few pounds or find someone in the university's Chemistry department to dispose of it for me!

Primer

All paints start with two or three coats of primer at a thickness of ~100-120 µm per coat. The period between coats is the 'flash-off' time. Most primers are mixed 3:1 primer:activator plus 10% thinners in disposable calibrated mixing cups bought from paint suppliers. Primers are usually white or grey and serve to fill small surface imperfections, adhere strongly to the substrate, and form a uniform colour for the basecoat. Once cured, primer should be sanded with ~P400 sandpaper by machine or ~P800 by hand. Inspect the panel for pitting where air has been trapped in the cured primer and fill with a spot putty such as 3M Acryl Putty before sanding again.

Basecoat and Clearcoat

Following the primer are two or three coats of colour basecoat of ~15-25 µm per coat (note this is very thin). Basecoat is usually mixed 1:1 with thinner and optional activator to improve chip protection. Modern paint finishes are metallic, containing a colour pigment flakes of aluminium, and/or pearlescent, containing particles of mica coated in titanium dioxide that produce interesting effects under direct light. These thin, delicate layers need protecting with two or three coats of transparent lacquer (or clearcoat) immedately after the basecoat flashes of, at about 50µm per coat, wiping first with a tack cloth. Clearcoat is usually mixed 2:1 lacquer:activator plus 10% thinner. Manufacturers' datasheets usually provide very clear guidance on the mixing and application of their products.

2K paint can be cured at room temperature, at 60°C (140°F) or with an infrared heater. Curing times are listed in manufacturers' datasheets. I use a domestic infrared heater placed about 60cm (24") from the workpiece.

Finishing

A panel can be handled after overnight curing at room temperature. However, the paint continues to cure for many weeks following application. The painted surface is usually not glassy-smooth but slightly hilly which produces a 'shimmery' surface. Thickness, paint quality and curing temperature all contribute to the degree of this effect.

In order to flatten the surface, it can sanded with P1500-2000 sandpaper which will remove its shininess (this can be a painful process as you watch your hard work being seemingly destroyed in seconds!). Particular attention can be paid to specks of dust protuding past the surface of the clearcoat.

Sanding is an optional step that achieves a very flat surface, though the following 'compounding' step can alone give satisfactory results if done correctly. Compounding uses an abrasive paste such as the popular Farecla G3 compound, applied with a damp sponge with a polishing machine or a cloth by hand. This will remove any sanding marks and give a uniform surface. Note that a polishing machine differs from a sander in that it is geared to run much more slowly at 1500-3000 rpm compared with 4000-10000 rpm. Swirl marks from compounding can be polished out with a good quality polish or filled with a 'hand glaze'. Wax should not be used for at least a month following application as it forms an airtight layer through which the still-curing paint cannot breathe.

Step-By-Step Process

The process of preparing, painting and finishing can be quite daunting but it is within the reach of the hobbyist. Some of the steps become automatic after refinishing a few panels. The following list is a recapitulation of the points made in this page which are easier to follow.

  1. Roughen painted surfaces with P280 dry or P320 wet sandpaper or bare metal with P120 dry sandpaper.
  2. On panels requiring filler, flatten the metal as much as possible then apply a thick coat while avoiding too many strokes that can tear partially cured filler.
  3. Sand the filler with a coarse sandpaper (P60-80) to get a flat but rough surface.
  4. Hammer down any high spots revealed by sanding.
  5. Apply a second thin coat of filler and sand with P120 followed by P240.
  6. Mask off any panels not to be painted with masking paper or at least two layers of newspaper using quality masking tape.
  7. Degrease and wipe a tack cloth over the surface to remove dust.
  8. Apply 2-3 coats of primer, allowing adequate flash-off time between coats.
  9. Pour excess paint into an airtight container (NOT down a drain), clean the paint container with universal cellulose thinner and spray thinner until all paint is removed.
  10. Sand cured primer with P400 sandpaper by machine or P800 by hand.
  11. Fill and small surface imperfections with spot putty and sand flat.
  12. Tack cloth the surface, then apply 2-3 coats of basecoat, being careful not to apply too thickly.
  13. Clean gun.
  14. Once flashed off, wipe carefully with a tack cloth being careful not to damage the drying surface.
  15. Apply 2-3 coats of clearcoat and allow to cure at least overnight before handling.
  16. Clean gun.
  17. After 1-2 weeks, sand with P1500-2000, or skip and compound straight away with compounding paste by hand or machine.
  18. Remove swirl marks with polish or hand glaze. Do not apply wax for at least a month following application.

Conclusion

I have reviewed the equipment and usage steps for the hobby refinisher. Many steps have come about from trial-and-error and most refinishers have their own preferred techniques to achieve an equally good finish (I'll admit that reading a similar statement when I was first learning only added to the confusion!).

You will probably make mistakes and learn from them; hopefully you'll have already learned from mine. It is possible to achieve a professional-looking finish in the home workshop providing you use the observe the safety warnings, use paint materials correctly, remove ALL traces of dust and don't start the next stage until the curing time has passed.

Refinishing at home can be very rewarding and will save a lot of money even though the initial outlay can be quite expensive at around £600 for all the equipment and materials. You will also find you have many more 'friends' when they learn you can respray their cars for them.

Happy spraying!