Sanding Between Coats Of Boat Paint
Sanding between coats often gets neglected by unprofessional Boat Painters. But is rubbing down between coats of paint really necessary? In this post we will talk about why and when we sand between coats, as well as what materials we use. If the paint job is to look nice and last a long time, then it is essential that we rub down the paint at the right stages. Always get permission from the Canal and River Trust before carrying out work on your boat while in the water. The Floating Boatyard is fully insured and licensed with CRT. We have strict risk assessments in place to avoid damaging the environment.
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Why Paint Needs Rubbing Down Between Coats.
There are several reasons why paint needs rubbing down between coats. We normally do this after the undercoat has been applied to the boat. Primers and high build undercoats can be quite thick paints. This can lead to brush marks and other imperfections in the surface. There is also the possibility of dust settling in the paint as it is drying. This is especially a concern when we are painting the boat outside in the open. Sanding down all the surfaces after the undercoating stage will flatten out the surface and remove any marks left by settling dust. Sometimes we do this after the first top coat as the colour difference can help to better see the imperfections in the surface.
This surface has been sanded after the first top coat. All imperfection were easily visible, making it easier to achieve a good finish.
Sanding between coats can also be done during painting of the subsequent coats of marine gloss or coach enamel. This will help to further flatten out the paint for a mirror finish. It also provides better adhesion for the following coats. When painting canal boats in the open, we often get the sun shining on the finished coat. We normally start very early in the morning and work in the shade to avoid the heat, but the sun sometimes shines on the paint all the rest of the day. This can lead to the paint hardening out completely and setting too far for the next coat to properly hold on it. It is therefore vital for the longevity of the finished product that we rub down the surface before continuing work.
How Sanding Between Coats Is Done
The first thing we do before sanding between coats is choosing the right sandpaper. We only use quality abrasives from professional brands such as Festool Granat and Mirka Abranet. These either come as traditional type sandpaper or as netting. Either is fine, though the traditional sandpaper tends to be a bit more abrasive. This can be counteracted by using a finer grade. We always err on the side of caution when choosing a sandpaper for sanding between coats. It is better to take of too little than too much. We can always go over the surface a second time.
It is best to use a quality sander for rubbing down paint. We use professional Festool sanders with eccentric motion. These will create a truly smooth surface and random pattern. Cheaper random orbital sanders can leave a rotating pattern in the surface of the paint, which can then be seen through the top coat when the job is finished. Not only will this look ugly, it also creates areas where small amounts of rainwater and dirt can collect, leading to problems down the line. Small corners need to be sanded by hand. We always make sure to rub down the surface until it is dulled all over. The next coat of paint will not stick properly to any shiny areas.
Suitable Grades Of Sandpaper For Rubbing Down Between Coats
The right grit of sandpaper for rubbing down between coats varies between brands. We always test the paper or abrasive netting on a small and inconspicuous area. This could be under the gunwales or seat of the boat. We find P400 or P500 is best suited for rubbing down between coats of undercoat. This is can also be used for sanding the last undercoat before applying marine gloss or coach enamel. For sanding between top coats, we usually use P800 or finer grade wet sandpaper. We always make sure all the surface has gone dull before applying any more paint. This can take a while, especially with the finer grits of sandpaper.
After Sanding Between Coats
Once we have finished sanding between coats, the surface must be cleaned. The next coat of paint will not stick to a dusty surface. After using wet sandpaper on top coats, this can be done by rinsing the whole boat with plenty of water and then waiting for it to dry. This method is very effective in making sure that all of the dust is removed. We must ensure the boat is fully dried out before continuing to paint. The next coat of paint will not hold on a damp surface.
Alternatively, we can use a damp cloth. This needs to be rinsed often. We need to wipe the surface more then once to ensure all the dust is removed. Special attention needs to be paid in corners and hard to reach areas. Tack clothes can be used to remove dust after sanding undercoats and where introducing moisture is not an option. Once the surface is clean, the next coat of paint can be applied.