HYMER AKTIV – Roof Seal and Skylight Maintenance

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Roof Seal and Skylight Maintenance for HYMER AKTIV

The Hymer Aktiv’s composite fiberglass-reinforced roof is genuinely tough, but the factory sealant around the rear skylight is the Achilles heel of this otherwise excellent build — plan on addressing it at the 2-3 year mark before it becomes a water intrusion problem rather than a maintenance task. The roof also carries two Maxxair 7500K fan cutouts, factory solar mounting rail penetrations, and the Thule HideAway awning bracket bolts, each of which is a potential leak point that deserves inspection every season. This guide walks you through a full roof seal refresh, from cleaning through final coating, using materials that are compatible with the Aktiv’s fiberglass roof substrate. Budget a full weekend: day one for cleaning, inspection, and targeted sealant work; day two for the EternaBond tape reinforcement and EPDM coating.

Required Parts

Step-by-Step Instructions

Step 1: Set Up Safely and Protect Your Electrical System

Before you put a single foot on the roof, address the Aktiv’s sophisticated Victron electrical system. Shore power, solar input, and the Victron MultiPlus-II 1600W inverter/charger must all be properly shut down — not just switched off at the panel. Open the Victron Color Control GX display (mounted in the driver-side cabinet near the galley), navigate to Settings > General > Shutdown, and confirm the system powers down gracefully. This protects the VE.Bus communication network from voltage spikes if you accidentally contact any roof-mounted solar wiring. Disconnect shore power at the TT-30 inlet first, then use the main battery disconnect. Do not simply flip the Maxxair fan breakers and assume the roof is electrically safe — the solar rail wiring runs exposed in shallow channels under the rail flanges, and a dropped metal tool can bridge terminals. Tape cardboard over both Maxxair fan domes before walking the roof; the polycarbonate lenses scratch easily and a boot heel will crack them. Use a non-conductive fiberglass ladder, position it at the rear corner away from the awning arm, and keep a second person on the ground as a spotter. Work in shade or early morning — a sun-baked fiberglass roof reaches 140°F and sealant won’t cure correctly when applied to a surface that hot.

Step 2: Inspect Every Penetration and Map Your Damage

Walk the entire roof systematically before touching anything. Start at the front ProMaster cab-to-body transition seam — Hymer uses a factory bead of lap sealant here that often develops hairline cracks first. Move rearward along the driver’s side, checking each solar mounting rail bracket: there are typically six lag points per rail, and the sealant around these is thin from the factory. At the Maxxair fan flanges, press firmly around the entire perimeter with your thumb — any sponginess in the surrounding fiberglass indicates moisture has already wicked under the flange. The rear skylight is your highest-priority stop. The Aktiv’s skylight uses a two-piece ABS frame with a compression seal; look for sealant that has pulled away from either the inner or outer frame edge, chalky white oxidation along the bead, and any brown tide-mark staining on the acrylic dome itself, which indicates water has pooled in the channel. Check the Thule HideAway awning mounting bolts at the roofline — the four through-bolt penetrations use ProMaster body sheet metal as a substrate, not fiberglass, and galvanic corrosion accelerates sealant failure here. Photograph every crack, gap, and suspect area before you start removing anything; you’ll reference these photos during reassembly to confirm full coverage.

Step 3: Clean the Entire Roof Surface

Proper adhesion of every product in this job depends entirely on how clean your surface is — this step is not optional and not quick. Mix the Dicor rubber roof cleaner and degreaser at a 1:4 ratio with water in a 5-gallon bucket. Apply it across the entire roof with a soft-bristle brush, working in 4-foot sections from front to rear. The Aktiv’s fiberglass surface has a gel-coat layer that holds road diesel, oxidized sealant residue, and tree sap in its microscopic texture; give the cleaner 5 minutes of dwell time before scrubbing. Rinse thoroughly with fresh water — chlorinated city water is fine, but if you’re at a campsite with hard well water, follow up with a distilled water rinse around your penetration points to prevent mineral deposits under new sealant. Around the solar rail brackets, use a soft toothbrush to get cleaner into the tight angle between rail and roof surface. Do not use petroleum-based cleaners, acetone, or mineral spirits anywhere on this roof — they will attack the gel-coat and degrade any existing EPDM sealant. Once rinsed, allow the roof to dry completely — minimum 2 hours in direct sun or 4 hours in overcast conditions. Run your palm across the surface: it should feel almost squeaky-clean with zero oily residue before you proceed.

Step 4: Remove Old Sealant and Prep Penetration Points

Old sealant that is cracked, lifted, or separated must come off completely — never apply new sealant over failed sealant, as you’ll simply seal moisture in and mask the problem. Use the plastic putty knife set exclusively here; a metal blade will gouge the fiberglass gel-coat and create new leak pathways. Work the blade at a low angle, 15-20 degrees, and peel old Dicor sealant back in strips. At the rear skylight, you will likely find the original bead has separated from the outer ABS frame edge — this is the known failure mode on Aktiv models 2-3 years old. Remove all of it around the full skylight perimeter: inner frame, outer frame, and the corner joints where the ABS frame sections meet, because water loves those corners. At the Maxxair fan flanges, remove sealant from the top side of the flange lip completely; the factory often under-fills the forward edge of the flange, which is where wind-driven rain enters first. After mechanical removal, wipe all bare areas with a clean rag dampened with the Dicor cleaner solution to pull residue out of the surface texture. Allow 30 minutes drying time. Inspect the exposed fiberglass under old sealant for soft spots, delamination, or discoloration — any area that flexes under thumb pressure needs to be documented and may require professional fiberglass repair before sealing.

Step 5: Apply Fresh Lap Sealant to All Penetration Points

Load a tube of the white self-leveling Dicor lap sealant into your drip-free caulking gun and cut the tip at a 45-degree angle to produce a bead roughly 3/8 inch in diameter. Self-leveling Dicor is specifically formulated for horizontal surfaces — it flows slightly after application to fill micro-gaps that a stiffer sealant would bridge over and leave hollow. Start with the rear skylight because it’s your highest-risk penetration. Apply a continuous, unbroken bead around the entire outer perimeter of the skylight frame, then tool it with a dampened finger to press it into the frame-to-roof joint. Apply a second bead along the inner frame edge where it meets the acrylic dome channel. At the skylight’s four corners, apply an extra blob and work it into the corner joint — corners are where thermal cycling concentrates stress. Move to the Maxxair fan flanges: apply a full perimeter bead on both fans, paying particular attention to the forward (front-facing) flange edge. At each solar rail bracket, dab a generous covering bead over the bolt head and 1 inch onto the surrounding roof surface in every direction. At the front cab transition seam and the Thule awning bolt penetrations, apply continuous beads and smooth them. Allow Dicor to skin over for 30 minutes before the next step — it will fully cure in 24-48 hours but is rain-safe after 2 hours.

Step 6: Reinforce the Skylight and High-Stress Seams with EternaBond Tape

EternaBond RoofSeal tape is your insurance policy — it creates a waterproof, UV-stable membrane over the joints that are most likely to move with the van’s flex. The 4-inch width is ideal for the Aktiv’s skylight frame, and the tape’s butyl rubber adhesive bonds permanently to clean fiberglass. Cut a length to cover each side of the skylight perimeter, adding 3 inches of overlap at each corner. Peel the release liner back only 6 inches at a time as you apply — if the full liner comes off and the tape folds onto itself, the piece is ruined. Position the tape so it covers 2 inches of the skylight ABS frame and 2 inches of the surrounding roof surface. Once positioned, press firmly by hand along the centerline first, then work outward to the edges to push out any air. Immediately follow with the J-roller, applying significant downward pressure in overlapping strokes — this is what actually activates the full adhesion bond, and skipping the roller leaves up to 40% of the adhesive unactivated. Overlap corner pieces by 3 inches minimum. Apply EternaBond over the front cab transition seam as well, centered on the seam. Do not apply EternaBond directly over fresh Dicor that hasn’t skinned — wait the 30-minute tack period from the previous step. Once applied, EternaBond does not need to be painted over but should be included under your EPDM coating for a unified UV barrier.

Step 7: Apply EPDM Primer and Roof Coating for Long-Term UV Protection

The final coating step protects your new sealant work from UV degradation, which is the primary reason Dicor sealant fails in the first place — uncoated lap sealant on a south-facing roof can lose elasticity within 18 months in sun-heavy climates. Apply the Liquid Rubber EPDM/TPO primer first using your 3-inch roller, working in 4-foot sections from the rear of the roof forward. The primer is thin and penetrates quickly — one coat is sufficient, but ensure full coverage with no skipped areas around penetrations. Allow primer to dry until it’s tacky but not wet, typically 45-60 minutes in 70°F weather; cooler or more humid conditions require longer wait time. Stir the EPDM rubber coating thoroughly — the solids settle during storage and an unstirred gallon will give you inconsistent film thickness. Apply the first coat of EPDM coating with the roller using a W-pattern, keeping a wet edge and working toward the ladder. One gallon covers approximately 75-100 square feet at the recommended 20-mil wet film thickness; the Aktiv’s roof runs about 60 square feet of coatable area, so your gallon is sufficient for two coats with careful application. Allow the first coat to cure until it’s dry to the touch — minimum 4 hours — before applying the second coat perpendicular to the first for uniform coverage. Keep the roof unloaded and dry for 48 hours after final coat before reconnecting your Victron system and resuming travel.

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