Bondo Vs. Rondo

Today I want to take a minute and talk to you about two different materials commonly used in pepakura builds.


The first you might have heard of before, it's Bondo Body Filler. Bondo is a material with a consistency similar to peanut butter that once mixed with hardener sets in 5-10 minutes for an easily sand-able surface. Normally one spreads the Bondo on top of their pep and sands it
down smooth.

The second is Rondo,a mixture of Bondo Body Filler and Fiberglass Resin. It's name is derrived from the R in Resin and the "ondo" part of Bondo. Rondo can be mixed in various consistencies but mostly resembles a runny milkshake. Rondo is not spread on like Bondo is but is instead poured on the inside of the pep and slushed around for an even coat. Rondo is also stronger, but heavier than Bondo though it sands much easier than plain Fiberglass Resin. It sets quickly, not as quickly as Bondo, but with enough working time that you can easily cover an entire prop in just one or two mixes. With bondo you can be mixing countless batches until your entire prop is covered. Rondo can also be used to fill smaller areas and make thin points less prone to snapping off. It can also be drilled into so you can mount other parts easily.

Mixing Rondo is very easy it's:

• One Part Fiberglass Resin
• One Part Bondo Body Filler
•  One Crap-ton of Hardener from each

Since both Resin and Bondo are Polyester based the hardener for either would work. I find the more cream based hardener you add the quicker rondo will set. Rondo has a working time of about 15 minutes, but during this time you must be slushing the mixture throughout your pep constantly. You can make runnier mixtures by adding more resin than bondo and make thicker mixtures by adding more bondo than resin. This mixture is on the thicker side:

 

If you leave your Rondo mixture before it's set it will pool at the bottom of your pep. You can use this to your advantage and fill small points to prevent breaking or to make solid areas for mounting other parts.

You can also fill VERY large gaps with rondo by taping up the gap with masking tape and letting the rondo on the inside of the pep pool ontop of the tap, pull the tape off and you'll have a mostly flat area to sand smooth. 

You can also entirely fill smaller pep areas to make finished parts in a manner of hours.

However Rondo is not a replacement for Bondo, though it is versatile in it's uses. Bondo is an additive material whilst Rondo is a subtractive material. What that means is that Bondo is used to add ontop of your pepakura and then you sand smooth until you see your pepakura starting to poke through. Whilst Rondo is used beneath your pepakura and your pepakura is sanded off completely until smooth. A pepakura finished completely with Bondo would be bigger than a pepakura finished completely with Rondo. In this example the black hexagon represents your very polygonal pepakura. Whilst the red and blue represent the material added to create the final circular shape.

A mixture of both Rondo and Bondo seems to be the ideal choice on paper, but in reality it's not all that simple. It's knowing which material to use when that comes with experience. I hope this explanation is helpful to you in deciding what kind of material is best in your own builds.