Vacuum bag compaction is recommended when bonding end grain balsa to a laminate surface using epoxy resins, as vacuum bagging provides the most weight efficient bond line consolidation. The long open times available with epoxies also allow large areas of core material to be bonded at one time.

When using contourable end grain balsa, it is generally recommended to install the core scrim side up whenever possible.

LamPrep® or AL600/10® end grain balsa surface can be used with epoxies. The AL600/10® will minimize resin absorption.

Step 1: Laminate Surface and Core Preparation

Laminate surface preparation is important when bonding to a cured epoxy surface. If peel ply was not used, then the entire surface will require machine sanding with 80 grit paper to leave a mat finish without areas of gloss. If peel ply was used, then sand smooth raised edges and ridges that could hold the core material away from the surface. (If unsure, follow the resin suppliers recommendations and test bond to a sample section of the laminate prior to a full core installation)

Precut and pre-fit the sheets before bonding in place, particularly when vacuum bagging. The sheets should fit together with minimal gaps.

Step 2: Mixing and Applying the Adhesive

An epoxy adhesive can be blended from a suitable epoxy resin thickened to non-sag consistency using a thyrotrophic modifier such as Caboose or Aerosol. Note: Due to the high strength of the balsa end grain, a low density modifier, such as Micro Balloons, which might be used to bed a low density foam core, could result in a glue line that would fail under stress before shearing balsa.

Use a Baltek Trowel at approximately an 80 degree angle to the surface.

Flat surfaces - apply the adhesive in a uniform thickness of approximately 0.04-inch. (1-mm)

Curved surfaces with scrim up. Areas of high curvature will use more adhesive to fill open kerfs. The amount can be determined on site and troweled in place as required. Core materials over 1-inch thickness may require additional adhesive to be applied to open kerfs of balsa sheets, after resin coating, while the sheet is back curved over a drum.

Filling the kerfs improves the integrity of the core layer and prevents possible moisture collection if the skins are ever compromised.

Male molding or scrim down core application does not require filling of open kerfs at this time.

Step 3: Priming the Core

With larger applications, particularly when vacuum bagging, steps 2 and 3 can occur concurrently to fully utilize the available working time of the epoxy.

Pre-coat one side of the end grain balsa sheet with laminating resin just prior to installation. The average amount of resin to wet out a LamPrep® surface is 1.5oz (40-grams) per sq.ft. An AL600/10® coated surface uses 0.70-oz (20-grams) per sq.ft. The resin is generally hand applied with a short nap length or foam covered roller.

Female molding - Additional resin is required to wet out the kerfs of Contourkore® end grain balsa sheets. Lay the sheets over a large drum and use a laminating brush to coat the open kerfs. Turn the sheet 90 degrees to coat the open kerfs in the other direction. Coating the open kerfs will help the epoxy adhesive to flow in and fill the kerfs during the bonding process. (Cover the drum with a plastic cover to prevent paint transfer and allow easy clean up.)

Male Molding - If bonding the core with the scrim side down there is no need to wet out open kerfs at this time.

Step 4: Core Installation

Contact molding - will require the end grain balsa sheets to be positioned and then bedded using metal compaction rollers with moderate pressure to bed the sheet evenly into the epoxy adhesive and fill any open kerfs. It is important that no voids remain between the laminate and the core.

Avoid excessive pressure or walking on the core before the adhesive has cured. Over-squeeze out of the adhesive can occur resulting in a localized dry bond.

Fillet strips or a low density epoxy filler can be used to close out the edges and fill any gaps. Fillet strips can be resin primed and bonded into place at the same time as the end grain balsa core.

Vacuum bagging - The open time of the epoxy adhesive, which varies with film thickness and workshop temperature, will determine the amount of working time available when vacuum bagging. The vacuum bag must be sealed and evacuated before the epoxy adhesive starts to gel. First pull 10 - 15 inches of mercury for initial compaction, then after a few minutes reduce the vacuum reading to 5 inches of mercury until the resin has cured.

Step 5: Inside Skin

The surface and edges of the end grain balsa core should be smooth with no ridges. Fill any voids in the core layer. Sharp corners should be rounded for laminate to run over without lifting or crimping, and any steps tapered for a smooth transition.

Male molding - open kerfs can be filled at this time with a low density epoxy paste and sanded smooth.

Prime the surface of the end grain balsa core with a coat of laminating resin just prior to applying the laminate.