Using Drywall Spackling Paste as a Filler Material

A comparison of 10 products.

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Spackle Brands Tested

A DAP DryDex
B 3M Patch & Primer
C DAP Fast-n-Final
D ACE Lightweight Spackle
E Patrick’s Model Magic Filler
F Smart Non-Shrink Wall Fix
G Sherwin-Williams Shrink-Free
H DAP Wall Board Joint Compound
I Red Devil Onetime
J Red Devil Pre-Mix Vinyl

For many years I have used lightweight drywall spackling paste as a filler material on wood and foam for model building, as well as an occasional homebuilt part. Lightweight spackle is much easier to apply right out of the container and easier to sand than epoxy micro. In recent years some spackle manufacturers have gone to vinyl-based formulations, and it became obvious that there must be a “best-brand” product for our application.

Samples for density and penetration test.

With that in mind I scoured local hardware stores and lumber yards and collected 10 different brands and formulations. Each product was spread on a section of pink foam in various thicknesses and allowed to thoroughly dry.

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After drying, an initial inspection revealed several samples where the spackle had shrunk sufficiently to crack the surface. Obviously, these brands would not be suitable for use in thick sections.

I also measured equal volumes of each product in identical containers. Each was weighed when prepared, and after a long drying time weighed again to determine the density of each sample. In addition, I placed each sample on a scale and pushed an awl point to the same depth in each dry sample cup, recording the force required.

Penetrator force measurement.

Indentation made by penetrator.

The foam test pieces were taken to one of our EAA chapter morning coffee sessions, along with a supply of identical sanding blocks and an evaluation sheet for each. After much dust production and discussion, I had a very non-scientific evaluation of the sanding characteristics of each sample.

The morning coffee group at work sanding spackle samples.

After the group had sanded all the samples smooth, I applied a generous coat of Rust-Oleum gray primer and let it dry overnight. Each sample was then machine sanded on the side to expose the primer layer thickness. The samples were photographed and the primer layer thickness measured. The primer layer varied between .003 and .011 inch across the sample set, and the thickest and thinnest are shown in the photos on the right.

Applying primer to all samples.

Most of the primer layer thickness is a result of the wet primer soaking into the spackle. This phenomenon was discussed with several builders at Oregon’s Independence Airpark, and universally it was felt that any primer, paint, or resin applied over lightweight spackle would thoroughly soak into the spackle. This turned out not to be the case with any of the 10 brands tested. This results in a great increase in the spackle’s skin toughness, but no improvement to the shear strength of the spackle below the primer layer.

.011-inch thick primer layer.

.003-inch thick primer layer.

Distance Rust-Oleum gray primer soaked into the spackle.

Sherwin-Williams Shrink-Free Spackle received top ranking, especially for applications where weight is an issue.

Penetration force and sample density of all products included in the test.

Force required to penetrate each sample to a set depth.

The group sanding test data, along with my density, hardness, and primer saturation findings, were tabulated and the results are shown in the charts. Here are the conclusions:

1. For applications where weight is an issue Sherwin-Williams Shrink-Free Spackle is the best product. It sands well, without balling up as some products do, and is one of the stronger of the lightweight products. In addition, the product does not crack in thick sections (see #2).

2. None of these products should be applied in large areas or thick sections (greater than .010 inch) under glass or carbon fiber due to their relatively low shear strength compared to composite materials. Using thicker spackle layers in these applications is to invite delamination. The overall strength of thin sections (less than .010 inch) will be improved by finish coatings.

3. For non-flight application such as molds or foam parts, a heavy product such as DAP DryDex would be a good choice as it is easy to sand, very strong, and doesn’t tend to crack in thick sections.

4. There was no advantage found in using the more expensive Patrick’s Model Magic. This product is typical of several available at hobby shops and is intended for balsa and foam model builders. The product was harder to sand and offered no strength advantage over other lightweight spackle brands.

Complete data for all spackling pastes included in the test.

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