Engine cowl part 4 (Master Model).

After the Cheek Cowl foam masters were bonded in place, a single ply of 8 oz fiberglass was laid up over the foam cores. light "Milk"coats of Bondo (Automotive Body Filler) are spread over the fiberglass. I use the term 'Milk Coat" for this body filler layer. This refers to a thinned out coat of filler. MEK or Acetone is premixed with the raw filler to a consistency of heavy pancake batter. This flows very well and fills the surface cleanly. here is a little trick I use when mixing Bondo, this is a good excuse not to throw away all of those free phone books. 

Phone Book mixing Palette

Every time you mix bondo you just tear the old page out. An excellent model maker at Johnson Controls in Holland Michigan showed me this trick. So far on the Robin project I have used 1.5 phone books. If you are thinning out the bondo, you make a pool in the middle just like you are spooning Gravy at Thanksgiving!!!

I use professional grade Bondo available only at Automotive supply houses, I stay away from the popular Bondo brand available at places like Auto Zone.

As the surface is sanded smooth, a final coat of Lacquer based 'Spot" putty is squeegeed on to the surface. This is a lower viscosity, air drying product designed to fill coarse sanding scratches and minor depressions. there is a methodology used when filling and sanding the surfaces. The cheek cowl was first coated and sanded flush and flat. Long flexible sanding boards are used for this step. These products are all available at professional Auto body supply houses. My favorite is Sherwin Williams automotive paint stores. Once the cowl and cheek cowl surfaces were sanded flat and wave free, I made a "drag" or radius sweep from scrap plywood with .75" radius. This is also the radius of my foam circular sanding board. Bondo was swept between the two surfaces. A small chisel shaped wood scraper was used after the bondo started to cure to remove away the excess that escaped from the sides. Get to know your Bondo product, you will be able to judge the correct time to sand, scape or surform plane. As I finished sanding the surfaces I would sweep on the spot putty. I usually let this cure for 24 hours before I actually finish sand the putty. As you progress from step to step finer and finer sand paper is used.

Front view of cowl during shaping

Another view at this stage. The L/H cowl has been roughed in and the inlet lip is still square shaped.

Inlet lip "Drag"

This is the technique I used to form the leading edge inlet lip. The tape was added as a guide . Bondo was mixed and swept into position. This is a long and laborious process. It took well over 5 hours of sanding and re sweeping to get this just right. Thank God for Sirius radio in my shop. This is a lot of work, I will admit this, but done correctly the results are beautiful. The anticipation of the final installed cowl, is really what keeps me going. This is truly a labor of love. Thankfully these cowls will be premolded for the plans builder. Not saying you can't do this yourself, but you will spend more money for Bondo and supplies than what I will eventually sell the cowls for.

Inlet during forming

add bondo, sand Bondo...and so on and so on. Until its perfect!!!!

The Master Spot Putty was sanded down with 180 grit. Then it was removed from the fuselage. This was easier than it sounds. It took a while to finally get the master off. I was able to get it removed with very minor damage.The next step was to construct a holding fixture that would hold the master vertical and horizontal. I want the master vertical so I can pour a plaster insert into the L/H cowl inlet. An insert is needed because any "splash" or cast of the master would be die locked in the area of the inlet. There are 4 more steps involved before the final layup mold is built. Because the cowl is designed to split through the cheek cowls, I want to have a joggled flange or "rebate" set into the surface of the lower cowl so the upper cowl sits flush. In order to do this, a master splash will need to be taken of the entire cowl . After the main splash is made, pattern wax of the desired thickness is set onto the master along the predetermined trim lines and another splash is made that reverses the contour. A second splash for both the upper and lower cowl will have to be made, From these splashes, the final lay up mold will be made.

holding fixture

This is the holding fixture I designed and built. The two perpendicular surfaces are designed so I can rotate the master 90 degrees. The top surface of the fixture will bond to the foam inside the master. I used generous scoops of Bondo for this

Fixture rotated 90 degrees.

its important to plan the work height when building fixtures. I will spend hours sanding this tool and econometrics are really important. 

Master installed on frame

The front of the master will need to be closed up. As you recall I set a 1/4" foam spacer behind the spinner master, this was so I could add the 1/16th inch spinner backing plate to close this off and decrease the gap to 1/8th inch between the spinner and the cowl

The next step was to shoot the master with a high solids filling primer. This is a catalyzed product that cures in about 3 hours. Really thick build ups can be accomplished with this product. The main splash will start after this. That is the subject of the next blog.