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Hello, I'm taking a dive into building wooden kits, I've completed a Quality-Craft semi-truck trailer, and I'm starting a pair of Quality-Craft bulkhead flatcars, but I'm not sure when to use sanding sealer. The instructions for the flatcars mention to use sanding sealer on the wood decks, but no mention of using it on the "steel" frame, which I have shaped from the included wood stock, but still haven't glued them. The trailer had no mention of sealer, only what colors of paint to use. Thanks!

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Generally use sanding sealer whenever wood is not supposed to look like wood, eg. sheet metal, steel, etc.  It seals the wood and raises and stiffens the grain so you can sand it to a smooth appearance.  It also makes the paint look more uniform especially if you want wood, plastic, and metal parts to look the same.

If wood is representing wood, I generally don't use it unless the grain of the wood is too coarse.  I want the grain to be visible.

Bob

I do what RRDOC does.   If I have wood parts that are supposed to be steel, I use sanding sealer or something similar and sand a bit.    On small parts a varnish might be easier to work.    the problem with wood representing steel is that the grain often shows through unless it is some very fine grain wood.    If the wood is supposed to be wood, I just use a few coats of paint and let the grain show.    

I have done old Walthers passenger cars and finished the roofs with sanding sealer.   After a few tries, I was able to get the wood roof to look like plastic or steel.     On those it did require a few coats of sanding sealer and a few sanding sessions but the result was well worth it.

Basically in many cases you don't want the steel parts to look like wood, so some sort of sealer can help with that.

Interesting to see how different hobbies use products for different purposes.  I’ve been a woodworker for over 30 years and during that time refinished more pieces of furniture than I care to count. In reading the responses, sanding sealer appears to being used as a grain filler.  Essentially sanding sealer is a diluted varnish with an additive that acts as a lubricant in the process of sanding.  Due to the small pores of most woods used in this hobby, it will work for that purpose.  If the wood has large pores such as oak and walnut, there are better products under the grain filler category.

In painting, the primer can fill a similar role where it can be sanded to smooth the surface and still allow strong adhesion of the top coat of paint.

Sanding sealer is primarily invented for production setting where you want to speed up the finishing process of stain then a spray on topcoats.  The quick drying properties and its ability to seal the freshly stained wood provides more consistent results.  It also has some elasticity properties that help keep the more rigid spray topcoats such as lacquer from failing over time.  All woods move with humidity.  The bigger the surface area, the more it moves, and some types of wood moves more than others.  A rigid topcoat will eventually crack and flake.  Sanding sealer remains elastic and reduces the impact of wood movement on the lacquer surface.  Many people use it on floors which is a bad idea because polyurethane finishes do not harden as much over a coat of sanding sealer due to its elastic properties.

All that said, none of that applies to this hobby.  The small pieces of  wood does not move much and we sure don’t walk on our models, so I see no big negatives in using it as a grain filler.  But, I can also see where often it would not be necessary at all. You would often get the same results by just sanding with very fine sandpaper prior to painting.

For pieces you want to look like wood, such as flat car decks, its diluted varnish qualities can act as a reasonable topcoat, especially since it dries flat unlike some other varnishes.  Some people use it under stain as a diluted sealer to prevent blotching, which is common with the types of softwoods used in this hobby.  But, there are better options such as a thin cut of dewaxed shellac.  Zinser Sealcoat is a good of the shelf example.

i often say wood finishing is part combination of art, science, and magic.  Sometimes products will be used in a way not intended, but works well anyway.  My only suggestion is the same I give in all finishes.  Test your plan on a scrap piece of wood first.  See if you like the results before trying it on the actual model.

@jstraw124 posted:

Interesting to see how different hobbies use products for different purposes.  I’ve been a woodworker for over 30 years and during that time refinished more pieces of furniture than I care to count. In reading the responses, sanding sealer appears to being used as a grain filler.  Essentially sanding sealer is a diluted varnish with an additive that acts as a lubricant in the process of sanding.  Due to the small pores of most woods used in this hobby, it will work for that purpose.  If the wood has large pores such as oak and walnut, there are better products under the grain filler category.

In painting, the primer can fill a similar role where it can be sanded to smooth the surface and still allow strong adhesion of the top coat of paint.

Sanding sealer is primarily invented for production setting where you want to speed up the finishing process of stain then a spray on topcoats.  The quick drying properties and its ability to seal the freshly stained wood provides more consistent results.  It also has some elasticity properties that help keep the more rigid spray topcoats such as lacquer from failing over time.  All woods move with humidity.  The bigger the surface area, the more it moves, and some types of wood moves more than others.  A rigid topcoat will eventually crack and flake.  Sanding sealer remains elastic and reduces the impact of wood movement on the lacquer surface.  Many people use it on floors which is a bad idea because polyurethane finishes do not harden as much over a coat of sanding sealer due to its elastic properties.

All that said, none of that applies to this hobby.  The small pieces of  wood does not move much and we sure don’t walk on our models, so I see no big negatives in using it as a grain filler.  But, I can also see where often it would not be necessary at all. You would often get the same results by just sanding with very fine sandpaper prior to painting.

For pieces you want to look like wood, such as flat car decks, its diluted varnish qualities can act as a reasonable topcoat, especially since it dries flat unlike some other varnishes.  Some people use it under stain as a diluted sealer to prevent blotching, which is common with the types of softwoods used in this hobby.  But, there are better options such as a thin cut of dewaxed shellac.  Zinser Sealcoat is a good of the shelf example.

i often say wood finishing is part combination of art, science, and magic.  Sometimes products will be used in a way not intended, but works well anyway.  My only suggestion is the same I give in all finishes.  Test your plan on a scrap piece of wood first.  See if you like the results before trying it on the actual model.

Thank you for posting this. Speaking as an experienced woodworker myself, your post is one of the most lucid explanations of a woodworking concept that I've seen on OGR (aside from my own posts, of course ).  The amount of woodworking and finishing misinformation and old wives' tales, especially related to varnish, is astounding.  Your explanation is spot on, complete, and concise regarding the purpose of sanding sealer and other options. 

As noted above, most any topcoat (primer, paint, clear coat) followed by sanding when dry will smooth out the grain.  IMHO if you want wood to not look like wood, any of these will look better than a single coat of paint on bare wood.
Some modelers will wet the wood to raise the grain then sand when dry. This works as well, but I worry about warping.

Bob

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