Sunday 25 September 2022

Upper Hull | Sanding the Primer

While painting the lower hull and the rudder I learned a lot about how to paint a boat, mostly by trial and error. The main lesson was that the condition of the surface of the primer determines the final finish of the boat.

If there are thin patches where the primer has been sanded back too much or insufficient paint has been applied, they will show through the gloss. No amount of gloss will cover such patches. The gloss only provides colour and shine, and it needs a deep and even coating of primer to do its job.

Equally, if the surface of the primer has blemishes or brush marks they will show through the gloss. In fact, the shiny nature of the gloss paint merely amplifies the effect. It looks awful in daylight. The primer was applied by roller and tipped out with a foam brush, so it is quite smooth but still shows brush marks.

So, it was really important to make sure the primer on the upper hull was entirely flat and smooth, with even depth of colour overall.

First the entire upper hull was sanded with P150 or P180 grit to substantially remove the brush marks.

Here we are sanding the cockpit deck.

I quickly discovered that really good lighting is essential for this process, otherwise it is impossible to see brush marks and other blemishes. The direction of the lighting has to be perpendicular to the brush marks to show them up by casting a minute shadow, as in the above photo.

I found I sometimes had to go to ridiculous lengths to achieve this, especially when sanding vertical surfaces. Here I have suspended the work light above the deck when sanding the footwell sides. 

Here the work light shines down on the forward deck while I am sanding the Dorade box sides and forward cabin wall.

I made rubbers from foam and cardboard tubing to sand the fillets. Here they are.

They worked well. Here I am sanding the fillet on the rear cabin wall.

In this photo I am sanding the sides and the front of the companionway hood.

I clamped the work light to a piece of scrap and balanced it above the hood on a box to get good lighting on the vertical surfaces.

Inevitably I sanded away too much primer in several places.

As stated earlier I learned to my cost when painting the bottom of the hull that these would show clearly through the gloss as a slightly different shade of colour, very obvious in natural light.

So, they all had to be properly covered. I marked each patch with a piece of tape. Like this.

It's impossible to keep track of where all the patches are if I don't do this. Another lesson learned from painting the bottom.

The patches were all painted over and gently sanded back with a P220 grit until they disappeared.

Lastly the whole upper hull was sanded for a second time with a P220 grit to completely remove any vestigial brush marks.

It now looked like this viewed from the bow.

 And from astern.

That's as good as I can make it, so will hopefully provide a good substrate for the gloss.

Getting there, slowly but surely!

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