SP 2424 (Part 2) - Roof A/C Hatch Installation

The next step in upgrading the SP 2424 is installing a roof hatch for the A/C system.  Many SP cars were upgraded in the early 1950s and by the mid to late 1950s the design of the roof hatch had changed to a "high" style one with a combing or flange around the opening into the A/C equipment to keep water and snow out.  The later style was also easier to use with a set of butterfly doors that hinged away from the car's centerline.

Here's the link for those of you that missed SP 2424 (Part 1) - Truck Conversion.

OwlMtModels #4001 SP Roof A/C Hatch (Brass) added to SP TTG artic chair car - Boyd Reyes Photo

The Parts - 3D Printed to Order!

The company I work for was asked by a customer to produce these hatches for some of his models.  They are now available in 3d printed lost-wax brass single casting (OMM #4001) $21.88 or a pair of hatches in High-Def Acrylate (HDA) plastic (OMM #4002) $12.95 from the Owl Mountain Models storefront at Shapeways.  The OwlMtModels website is also updated with a new 3D Printed Detail Parts page.

OwlMtModels' 4002 SP Roof A/C Hatches 3d printed in HDA plastic for $11.95 for a pair of hatches.

One point should be made here is that each set of parts is printed or cast as a one-off part, not mass produced, so this is reflected in the higher cost per unit for these "Printed-on-demand" parts.

The backs of the hatches have the remains of the support structure for printing, this will be easily sanded away.

I ordered a pair of the OMM #4002 plastic parts which arrived in about 12 days from the printer in New York.  The HDA material was still slightly soft, so I planned to clean them up and then let them UV cure in the sun for another 30-60 minutes with rubber bands holding it to the car roof.  This would hold the thin parts in the correct shape for the SP 2424's roof once the material fully cured.

If I just left the part to fully cure in the sun without being rubber banded to the car, I'm worried that the part will cure in a warped or twisted condition.  Once cured, in my experience, trying to fight the "set" that a part's taken is a real problem.  In a way I'm glad the parts came slightly too soft, so I can be sure to have them cure in correct alignment with the car's roof.

In this photo the bowing of the parts can be seen.  This will be corrected during pre-installation.

The next step with the soft hatches is to file smooth the bottom of the hatch so it will fit snuggly down onto the SP 2424's roof.

Back side of Hatches with support material to be removed

The bottom of the hatches needs to be cleaned up somewhat.  I did it with a small file.  The material was still soft enough that the parts flexed nearly flat.  Supporting the top of the part while doing this is a good idea.  Also make sure not to take too much off and change the contour of the part.

Here's the partly filed down underside.

I also filed away the sprue that was connecting the two parts from the side of the part.  I apparently slightly beveled the last 1/8" or so on one side of these hatches during this cleanup process.  The master file at Shapeways has been updated to fix this possible issue.

Here's the smoothed Hatch bottom.

Modifying the SP 2424's Roof

The new hatch will need to sit flush with the roof of the SP 2424.  There are two roof ribs in the way of doing this.  I also need to mark the centerlines of where the hatch will go.

Using my mechanical pencil I mark where the centerline of the roof is.  This will match up with the centerline joint in the roof hatch.  I held the hatch in place and marked around the hatch with my pencil.  This will create a mark about 0.015" away from the hatch edge.

Here's the roof marked with pencil and Sharpie.  The hatch is laying off to the side also.

I used a Sharpie to make the markings inside the lines darker for the camera to see.  The next step is to carefully cut the ribs down.  They need to be removed almost all the way out to the lines.  I marked the edges with an Xacto #11 blade, scoring across the ribs, then used a file on the portions within the lines to quickly get down even with the roof.

Here's the roof after filing is complete.

If the ribs are slightly too short, it shouldn't be noticeable when the hatch is in place with the slight overhang on all sides.

Sun Post-Curing

The next step was to let the parts UV cure a bit more in the sun.  I used a rubber band around the car body and a piece of styrene strip to rest the truck frame level against, so as not to damage the truck bolster assembly built in SP 2424 (Part 1).

After about 30 minutes in the morning sun, I moved the rubber bands over to allow the previously covered areas of the hatch to cure some more.

I considered the parts done when I couldn't scratch the surface of the hatch with my finger nail anymore.  The part is still somewhat "spongy" but I believe this is from the part only being 0.030" or so thick.  I still treat the part somewhat carefully as I don't want to "crush" the arch in the part which will match the roof contour.

Sanding Prep

Generally 3d printing does require some cleanup on the parts, rather the same way that injection molded parts sometimes have flashing and injection gate marks.  I started by cutting the two hatches apart with sprue cutters.  The "stepping" from the printing can be seen in the top surfaces of the hatches.  This looks rather like wood grain.  Not much can be done on the top of such a large radius arc to prevent this from happening.  It is nice to see that the brass OMM 4001 parts had very minimal stepping.

The brass OMM 4001 parts have minimal stepping which painting easily covers.

With the HDA plastic parts now cured more, I felt better about doing some sanding.  I primed one of the two hatches with UP Harbor Mist Gray to see how bad the stepping was.  The second hatch I started with sanding down before priming.

There was some stepping so I did some light sanding.

It looks worse than it is, but I decided to do some light sanding anyway.

Primed hatch sanded around the handle details.

The sanding was done with a 600-grit foam sanding stick.  This only took a couple of minutes.  I was careful to brace the part well as it was still very flexible even after curring more in the sun.

Painting the Hatches

I painted the bottoms edges of the hatches first from below.  Then painted the tops of the hatches.

Here are both hatches after painting.  The left one looks like I need to rescribe the center panel line again.

I installed one of the hatches on the SP 2424 with ACC.  The HDA material likes ACC as it quickly bonded the hatch to the roof.  The slightly rough bottom of the filed hatch and filed mating area on the car roof probably helped this.

Here's the hatch installed.

Touching Up the Roof Paint

The next step was to touch up the roof of the car.  I'd left the pencil marks extended out past the end of the hatch so it was easier to line up.  Also I had some light weathering on the car roof which I wanted to paint over for a fresher shopped look for the whole car.

Here's the Hatch installed and waiting to touch up the roof to cover the pencil marks.

Masking was done with blue tape.  I didn't worry too much about covering and sealing down the edges, as I planned to shoot the Harbor Mist Gray from a low angle.

SP 2424 masked and ready to touch up the gray roof.

I carefully removed the masking from the car sides.  One of the hand grabs next to the door decided to choose that moment to pop out of the holes and the short nib ends were too short to easily slip back in.  I just replaced it with a section of 0.014" steel wire.  Reviewing these pictures also reminds me there's a ladder rest grab missing on the roof.  I'll have to replace that at some point.

All Shopped and Ready to Go!

Detail view of the finished and painted Hatch.

Remember what the car looked like before the upgrades?  Here it is with the original Athearn-Genesis "Triple-Bolster" trucks.



Here are the current side views of SP 2424 with the new Walthers GSC trucks and OMM roof hatch installed.  It certainly looks a bit different now with the new trucks and extra hatch on the roof.

Left side SP 2424

Right side SP 2424

I should mention in closing roughly how long it took for me to do each of the steps;
Part sanding and prep (2 hatches) = ~10 minutes
Modifying the roof of the car (marking & removing ribs) = ~10 minutes
Rubber banding parts to roof and UV Curing = two blocks of 30-45 minutes (unattended) - Note this was doubled for curing the second hatch because I only modified one car roof.
Pre-Painting Hatches off car (2 hatches) = 10 minutes (or less) with air brush
Mounting Hatch to car (1) = ~3 minutes
Masking and airbrushing whole roof to blend new and old colors = ~15 minutes

Total time spend = ~ 1.25 hours working time, plus UV curing time.  It is certainly easily done in an enjoyable few short afternoons with drying times in between.

This wraps up Part 2 of my SP 2424 blog post.  In the next part SP 2424 (Part 3) - Radio Antenna Stanchions I cover more on detailing the roof.

Jason Hill

Related Links:

SP 2424 (Part 1) - Truck Conversion

OwlMtModels Shapeways Storefront

Index Page for SP Lightweight Passenger Car Models - HO

SP 2436 (Part 1) Upgrading Athearn-Genesis 77-C-3 Chair Cars