Priming Below the Waterline

[drjeckl]

I finally got to prime the hull below the waterline on my Lady Nelson.  I thought I had a pretty tight second planking but the grey primer shows that there were some gaps between planks, but it shows that the walnut planks provided with the kit are very porous.  See below:

 

  

 

    

 

Two questions:

1. Is a second coat of primer needed?
2. Should I fill in the gaps in the planks with filler?  And then prime again or will the final coats of white be enough?  I'm interested in seeing the wood through the waterline painting but not sure how it will look.  Any appropriate photos out there?

 

Thanks...jce

Edited September 5, 2021 by drjeckl

[Jim Rogers]

If it were me I would fill, sand, prime again. It looks good now and will look even better when painted.

[Roger Pellett]

It looks like your planking job gave you a nice fair hull- Well Done!

 

I would fill any large nicks and gaps with a sandable filler, one that will completely harden.  I would not try to fill the pores in the wood.  Then sand thoroughly.  Many modelers consider the first coat of primer to be sacrificial. Then prime again. It may take more than one additional sand and prime cycle to get the results that you are looking for.

 

Roger

[thibaultron]

Tell everyone you super detailed it, including Torpedo Worm damage!😉

[Bob Cleek]

The distinction between primer and sanding undercoat is often overlooked. Primer is thin and soaks into the bare wood surface. It can be thinned paint or shellac. I prefer shellac as it is thinned with alcohol and dries quickly to a hard coating that sands easily while penetrating the wood well. For those using water-based acrylics, you should not use a water-based primer on bare wood because the water will raise the grain on the bare wood surface. For this reason, among others, my standard primer is shellac. (Which is also cheap and readily available!)

 

Applying repeated coats of thin primer will take much longer to fill grain and other imperfections, as will applying repeated costs of finish paint. Sanding undercoat is a huge time-saver. Also note that filling imperfections with sanding undercoat has its limitations. It isn't intended for filling a 1/32" gap in planking. For that use surfacing putty (sometimes called "fairing putty",) which is a peanut-butter consistency acetone-thinned material similar to sanding undercoat that is used for filling larger imperfections. Surfacing putty should be covered with primer or sanding undercoat after sanding fair and before applying any finish paint because it may absorb some of the later-applied coating and create a visible difference in the later coat, often a "flat finish" that sticks out like a sore thumb on a gloss finish coat.

 

Once primed, a sanding undercoat is used to fill the small imperfections and grain. This undercoat paint is relatively thick and contains a fair portion of talc (chalk) which is what thickens it so it will fill the low spots and makes it very easy to sand. The undercoat, when fully dried, is sanded smooth. If some imperfections remain, another coat is applied and sanded again, and so on until the surface is perfect. Close examination under good light is preferred, and for final examination running the fingertips lightly over the surface is recommended because your fingertips are more sensitive for judging smoothness than your eyes.  Only after the surface is prepared in this fashion, should one apply the finish coats thinly until the desired finish is achieved. 

 

Oil-based sanding undercoat can be covered by water-based finishes because it is sufficiently flat to provide a mechanical bond, although if one is using  a water-based finish coat, it's safer to use water-based sanding undercoat. Oil-based paint and varnish are preferred for fine finishes due to their more durable harder surfaces which are easier to sand and polish. However, many prefer acrylics for their faster drying time and other more user-friendly qualities.

'

And the one rule that must often be followed and is rarely mentioned is to always, always, always, test every paint or varnish you are going to use on a scrap of the same material you are going to finish before you start applying it to the finished workpiece! This may be difficult for the impatient, but the additional time taken will ensure you avoid tremendous grief. Finishing is sometimes a mysterious process. Sometimes the smallest differences in the mixing of materials, their age, or the ambient environment can result in a failure, usually in adhesion or drying. It's often difficult to know why and frequently unpredictable. It's far better to know that before you've an applied incompatible or defective coating to an entire hull or to small, detailed parts that are nearly impossible to strip and start over on.

[druxey]

I think you meant Teredo worm, Ron: Torpedo worm damage is much more serious!