Balsa paint.

Yes, Balsa needs to be sealed. Some maybe more than others because porosity varies. Final paint will define sealant. Suggest one option is to consult any flying model enthusiasts that still build aircraft from balsa.

Wayne

As I recall from my model airplane days........

.......we used a product called "sanding sealer" prior to painting balsa.  This might not work for you as it needed sanding to smooth it out. It was basically some kind of clear "paint" with ultra sine wood particles in it.  Sanding that clapboard could be difficult.  
 

Funny story.  I built a semi scale model of a Chipmunk intended for a .23 motor. I got so involved on its finish with the sun bursts on the wings using lots of coats of sanding sealer and paint for a mirror finish that it ended up very heavy and would barely fly with the installed motor!  We had to re-motor it with a .40 motor and it flew like a champ, way to hot for me to handle so I sold it to my flying buddy who could handle it!  Back to model trains for me after that!

Peter

That's the conventional wisdom.

Yes, that is the conventional wisdom; seal it first.

  Why ? I do a lot of scratch

Why ?

I do a lot of scratch building with balsa . I do not seal or prime. The buildings are built, well braced, except for doors and windows and roof. I paint with cheap craft paints,brush or airbrush. Then install doors windows and roof.

I have NO issues. I don't under stand the need for sealer or primer

I'll test it both ways ...

Peter,  I've seen sanding sealer, but thinking I'll avoid anything needing sanding just so I don't wreck the detail on the siding.

Jim, and Sue,  I expected to get varied opinions and I can see both sides.  My gut is telling me to seal these panels but I plan to test painting both ways.  

I've been racking my brain for what to seal with and aerosol shellac is the best thing I can think of. I'll just mist it on very, very lightly and see where it goes.

If anyone sees a red flag with this type of sealer, PLEASE let me know!

Seal or not?

It depends on how you plan to finish the building. Do you want a new smooth paint look or an old weathered look with grain showing? or will it be stained with a semi transparent coat? You also need to consider how glue will affect the paint, glue first makes paint and stain not soak in the same as it does on unglued areas which is important to paint or stain appearance. In general if I plan to paint a building with a full coat of paint I usually wait and paint everything when it's done, but if there are lots of hard to paint details like window trims etc. I might paint them before gluing them on then give the whole thing a clear coat when done to blend it in. Probably the best way to figure this out is to build a few small balsa structures trying differnt methods. Dog house, out house, tool shed, etc. don't take much time or materials...DaveB 

ctxmf74,  Thank you for the

ctxmf74,  Thank you for the comment. I'd appreciate your input on this if you pick it up again.

I'm recreating my dad's farm house in O-scale, minus the bumpout seen on the left which was not part of the big addition made to the house in 1880.  The 1880 siding was wood planks in clapboard style painted white. I do not want a weathered/old look; I want it to look like freshly painted wood boards. Today the siding is vinyl, but considering the original siding was wood, in 1880 I can imagine the texture of the grain would have been visible beneath the white paint. I don't mind if the texture of the grain is visible on my model.  My assumption is that sealing my siding with the shellac spray prior to painting will help retain the texture of the grain, but I might have that backwards. 

As for painting it all at once, I don't think that's possible with my plans to assemble the structure. The second image below shows my wall framing for the first and second floors.  There will be second-story floor joists spanning the lower framed assemblies. Installing those floor joists will require me to install all the lower wall frames, install the upper floor joists, and then install the upper wall frames.  So, except for the roof trusses, I will have a completely framed house before any siding is installed.

I will use my wall frames as templates for locating/tracing the window and door openings on the back of the siding before installing the wall frames onto the floor.  Now, painting the siding: The siding panels I have are 6" wide which is more than enough to cover the lower and upper wall frames, combined, in a single piece. The third image below shows what one finished siding panel will look like. My plan is to cut the siding panels to size, cut the window and door openings in the siding, affix the window and door casings to the siding panel, paint the panels on both sides, install the window glazing and then attach the completed siding panels to the framed structure. Then all the corner trim and header board (also pre-painted) will be installed to complete the exterior look.  Then, any touch-up as needed by brush.

There will be no paint overspray on the wall studs or floor. I want to keep that dark contrast between the wall studs, floor, floor joists, and the inside of the white siding panels.  Why?  Because my design has a removable roof assembly so that one can inspect the interior framing!!   

Best regards,

Kevin

Sanding sealer, as used in

Sanding sealer, as used in most flying models, is clear butyrate "dope' with a powder filler of some kind. When used properly, you can get a sheet metal sheen on the balsa. My dad could do that.

I'm not sure about using that on/near plastics or other finishes, or as a base coat under other finishes.....  I believe a fully cured base coat would be OK under other finishes, in particular acrylics, but I'd want to test on scrap pieces before committing to it. 

It seems to me that Testors used to have a non-"dope" sanding sealer, too, but I don't know if they still make it,

  "As for painting it all at

Hi Kevin, Sounds like you have a good plan about how to proceed, and a neat structure by the way. When painting parts separately you just need to figure out what glue to use to make sure it's compatible with your paint. You also will want to minimize glue squeeze out of joints as it will change the color of the paint and can leave rough spots if painted over. For non structural parts it's not too hard to keep the glue to a minimal amount. Parts that are under load at times( like gondola sides to floor joint or freight car body bolsters etc)  need a bit more care in assembly. Before commiting to a paint/glue construction sequence for the house you could try a couple of mock up joints using the planned method to see if there are any snags....DaveB 

A new way to trim out the siding

Dave B,  Your words, particularly, "...when painting parts separately" brings up an idea that I've never really liked about my assembly plan - that being painting / attaching the vertical trim boards individually.  I've revisited that and came up with a better solution.

I want the vertical trim boards on the corners to appear to trap the siding just as it appears on the real house. I tested a dado cut on a small piece of siding to give me a groove to lay in a trim board. The trim board will be 1/32" thick.  I made the dado a little less than a 32nd deep so the trim board will just barely stand proud of the high points on the siding.  This all worked out pretty good.  Now I can attach my trim boards directly to the siding so the two get painted at the same time.

Just have to be careful not to flatten the raised edges of the siding boards due to running it back and forth across the saw blade.