HYMER AKTIV – Roof Seal and Skylight Maintenance

Roof Seal and Skylight Maintenance for HYMER AKTIV

Ignore a cracked skylight seal on your Hymer Aktiv and you’re not looking at a slow drip — you’re looking at saturation of the substrate, delamination that spreads silently through the roof structure, and a repair bill that climbs from hundreds into thousands before you even realize water has been pooling underneath. I’ve pulled into campgrounds to assess damage on Aktivs where the owner thought they were dealing with a surface crack, only to find the entire roof cavity damp and the plywood sheathing soft enough to press a thumb through. The problem isn’t one weekend of rain; it’s that small failures compound across months. A hairline fracture in the factory sealant around the rear skylight or a lifted corner of lap sealant at a solar rail bracket doesn’t announce itself loudly — it whispers through a musty smell, a ceiling panel that feels subtly spongy when you step near it, or nothing at all until the damage is already there. This guide walks through what I check on every Aktiv service call, what I replace, and the sequence I follow to lock moisture out for real instead of just patching the visible problem.

Required Parts

• Self-leveling Dicor lap sealant (white, 10.3 oz tube) Dicor 501LSW-1 EPDM Self-Leveling Lap Sealant, 10.3 oz – White
• EternaBond RoofSeal tape (4″ × 50 ft roll) EternaBond RoofSeal White 4″ x50′ MicroSealant UV-Stable Roof Repair Tape
• EPDM rubber roof coating – 1 gallon HENG’S Industries Rubber Roof Coating, 1 Gallon, EPDM Waterproof UV-Resistant
• Dicor rubber roof cleaner and degreaser – 1 gallon Dicor RP-RC-1GL Deep Cleaning Rubber RV Roof Cleanser Spray Refill, 1 Gallon
• Plastic putty knife set (for removing old sealant without tearing membrane) Rerdeim 5-Piece Putty Knife Scraper Set, 1″–5″
• Professional drip-free caulking gun Newborn 930-GTD Drip-Free Smooth Hex Rod Cradle Caulking Gun, 10:1 Thrust Ratio
• Liquid Rubber EPDM/TPO primer – 1 gallon Liquid Rubber EPDM and TPO Primer, 1 Gallon – RV Roof Basecoat
• 3-inch paint roller kit (for applying roof coating) ROLLINGDOG Small Paint Roller Kit, 3 Inch with Tray and Stir Bar
• J-roller (for pressing EternaBond tape to full adhesion) POWERTEC 12″ Long Handle J Roller, 1-1/2″ Diameter, 3″ Wide

Step-by-Step Instructions

Step 1: Set Up Safely and Protect Your Electrical System

Before your weight touches the roof, your Victron electrical architecture needs to be completely offline — not just powered down but safely secured against voltage transients. Open the Victron Color Control GX interface (driver-side cabinet, near the galley), navigate to Settings > General > Shutdown, and execute a controlled system halt. This prevents spikes across the VE.Bus network if a metal tool accidentally contacts any of the exposed solar wiring that runs in shallow channels under the roof rail flanges. Shore power disconnects first at the TT-30 inlet, then engage the main battery isolator. The Maxxair fan breakers are not sufficient isolation; solar conductor rails are live even when everything at the control panel reads offline. Cover both Maxxair domes with cardboard before you set foot on the roof — the polycarbonate is soft and one boot heel opens it up permanently. Use a fiberglass ladder (nonconductive), position it at the rear corner away from the awning articulation arm, and have a ground-level spotter positioned where they can see your feet. Timing matters: work in shade or very early morning because a fiberglass roof surface in direct sun reaches 140°F, and sealant applied to that temperature profile won’t cure to proper elasticity.

Step 2: Inspect Every Penetration and Map Your Damage

Survey the full roof without disturbing anything first. Begin at the front boundary where the ProMaster cab transitions to the body — Hymer’s factory lap sealant at this joint often develops hairline cracks as the primary failure point. Track rearward along the driver’s side and examine each of the six solar mounting rail brackets per rail: factory sealant applied here is consistently thin and vulnerable. At both Maxxair fan flanges, use firm thumb pressure around the entire perimeter — any depression or soft spot signals that moisture has already penetrated beneath the flange base. The rear skylight is your top-priority inspection zone. The Aktiv’s skylight sits in a two-piece ABS compression frame; look for sealant separation from either the inner or outer frame lip, white chalky oxidation streaking the bead, and any brown discoloration or tide-marks on the acrylic pane itself, indicating trapped water in the frame channel. Examine the four through-bolt penetrations securing the Thule HideAway awning base to the roofline — these fasteners thread through ProMaster body sheet metal rather than fiberglass, and dissimilar metal corrosion accelerates sealant deterioration here faster than anywhere else. Document each crack, gap, and suspect zone with photographs before you begin removal work; you’ll need these reference images to verify complete coverage when you’re reassembling.

Step 3: Clean the Entire Roof Surface

Every adhesive bond throughout this repair depends entirely on surface cleanliness — this step is non-negotiable and takes real time. Combine the Dicor rubber roof cleaner and degreaser at 1:4 ratio with water in a 5-gallon bucket. Distribute the mixture across the full roof using a soft-bristle brush, treating 4-foot work sections in a front-to-rear progression. The Aktiv’s fiberglass has a gel-coat layer that traps road diesel, oxidized sealant breakdown products, and organic weathering residue in its microscopic structure; allow the cleaner to dwell for 5 minutes before brushing. Rinse exhaustively with clean water — municipal supplies are acceptable, but if you’re camping at a site with high-mineral well water, follow the initial rinse with a distilled water flush at all penetration zones to prevent mineral salt deposits under new sealant. Around the solar rail brackets, use a soft-bristle toothbrush to drive cleaner into the recessed angle where the aluminum rail meets the fiberglass surface. Petroleum-based solvents, acetone, and mineral spirits must stay off this roof entirely — they will degrade the gel-coat and compromise any remaining original EPDM. The surface is dry and ready when your open palm slides across it with a clean, dry, slightly squeaky feel and zero oily residue remains — minimum 2 hours direct sun exposure, or 4 hours on overcast days.

Step 4: Remove Old Sealant and Prep Penetration Points

Cracked, lifted, or separated sealant must be completely removed — layering new sealant over failed material traps moisture underneath and simply prolongs the problem in disguise. Use only the plastic putty knife here; metal scrapers will gouge the gel-coat and create additional water pathways. Keep the blade angle shallow, 15-20 degrees, and peel old Dicor backward in long strips. At the rear skylight, you will almost certainly find that the original bead has separated from the outer ABS frame edge — this is the documented failure mode across Aktiv production. Strip all sealant from this full perimeter: the inner frame surface, the outer frame surface, and especially the corner joints where ABS sections meet, since water gravitates toward those interior angles. At the Maxxair fan bases, remove sealant from the top lip of the flange completely; factory application is frequently light at the forward flange edge, exactly where wind-driven precipitation infiltrates first. After physical removal, wipe every exposed area with a clean rag wetted with Dicor cleaner solution to pull embedded residue out of the surface grain. Allow 30 minutes drying. Inspect any exposed fiberglass where old sealant was bonded — soft spots, delamination signs, or discoloration indicate water damage and require professional fiberglass work before you reseal over them.

Step 5: Apply Fresh Lap Sealant to All Penetration Points

Load a tube of white self-leveling Dicor into your drip-free gun and cut the applicator tip at 45 degrees to create a bead diameter of roughly 3/8 inch. Self-leveling Dicor is engineered for horizontal surfaces — it spreads slightly after you lay it down to fill micro-gaps that rigid caulks would skip over and leave hollow. Begin at the rear skylight since it carries your highest water risk. Lay a continuous, uninterrupted bead around the entire outer skylight frame perimeter, then use a damp finger to work the bead into the frame-to-roof joint. Apply a matching bead along the inner frame edge where it borders the acrylic dome cavity. At each of the four skylight corners, deposit an additional generous blob and work it thoroughly into the corner joint where thermal cycling concentrates the most stress. Move to both Maxxair flanges: apply a complete perimeter bead on each fan, with special attention to the forward-facing edge — this is your first water intercept. At each of the six solar bracket locations, deposit a thick covering bead centered on the bolt head, then extend 1 inch onto the roof surface in every direction. Apply continuous beads at the front cab seam and the four Thule awning bolt penetrations and smooth them all. Dicor skins over in 30 minutes and is weatherproof after 2 hours, but full hardening takes 24-48 hours — you can proceed to the next step once it’s no longer tacky to the touch.

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

EternaBond RoofSeal tape is your secondary defense layer — it bonds a permanent, UV-resistant rubber membrane over the joints most likely to shift as the van flexes and moves. The 4-inch width is sized for the Aktiv’s skylight frame and the butyl rubber adhesive fuses permanently to bare, clean fiberglass. Measure and cut lengths for each skylight side, adding 3 inches of overlap allowance at each corner. Peel back only 6 inches of the release liner as you work — if the full liner releases and the tape folds onto itself, the piece is scrap. Position each length so that 2 inches covers the ABS frame and 2 inches covers the adjacent roof substrate. Once positioned accurately, press along the centerline with your hand first, then use overlapping strokes to work outward and eliminate air bubbles. Immediately follow with the J-roller, applying significant downward pressure in overlapping passes — this roller work is what actually activates full adhesive contact, and skipping this step leaves 40% of the bond inactive. Overlap all corner seams by 3 inches minimum. Apply EternaBond across the front cab transition seam as well, centered on the bead. Do not tape over Dicor that is still sticky; allow the 30-minute tack period from the prior step to complete first. EternaBond does not require paint coverage but integrates cleanly under your EPDM roof coating for unified UV protection.

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

The final protective coating shields your sealant work from ultraviolet degradation, which is the leading cause of Dicor sealant failure — bare lap sealant on a south-facing roof loses elasticity within 18 months in high-sun regions. Apply the Liquid Rubber EPDM/TPO primer using a 3-inch roller, working in 4-foot sections from rear toward the front of the roof. Primer is a thin film that absorbs quickly — a single coat provides adequate preparation, but coverage must be complete with no gaps around penetrations. Allow primer to reach a tacky state without being wet, which is typically 45-60 minutes at 70°F; cooler or more humid conditions extend this window. Thoroughly stir the EPDM rubber coating before use — the solids fraction settles during storage and unstirred product produces uneven film thickness. Apply the first coating with your roller in a W-pattern, maintaining a wet edge and advancing toward the ladder position. One gallon covers roughly 75-100 square feet at the specified 20-mil wet application thickness; the Aktiv’s coatable roof area is approximately 60 square feet, so your single gallon yields sufficient material for two coats with controlled application. Allow the first layer to cure to touch-dry state — a minimum of 4 hours — before applying the second coat at right angles to the first, which ensures uniform final coverage. Avoid foot traffic and keep the roof unloaded and dry for 48 hours after the final coat hardens before you restore power to your Victron system and return to travel.

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The Sealant That Actually Stays Put on Hymer Aktiv Roof Seams

The Aktiv’s seams and penetrations—particularly the skylight base and vent flanges—become water entry highways the moment the original sealant begins to fail. Silent infiltration is the danger; moisture advances underneath while you’re parked somewhere else, building mold and structural rot before the smell tips you off. Once you’ve scraped the old, failed sealant back to bare substrate, you need a product that will self-level into the seam, remain pliable as the rig shifts seasonally, and actually stay bonded instead of pulling away in year two.

What works

• Flows on its own into seams without feathering — the bead you apply settles and remains positioned, even across the Aktiv’s modest roof pitch.
• Retains flexibility throughout its service life; expands and contracts with the rig instead of becoming brittle and fracturing after seasonal thermal cycling.
• Tack-dry window is measured in hours rather than days, so you can coat over it with EPDM protection the same day, and it grips both metal and rubber surfaces without requiring an intermediate primer layer.

What doesn’t

• The white finish yellows and dulls significantly if left uncoated — it is not engineered to be the final, visible layer and will deteriorate in appearance and performance without a UV-stable topcoat.
• A single 10.3 oz tube applies approximately 15–20 linear feet of continuous bead depending on how wide you lay it; a complete skylight perimeter plus vent flange work will consume two or three tubes, so budget for that quantity upfront.

I second-guessed this product the first time I squeezed a bead into a deep seam and watched it settle—my instinct was to smooth it, which would have ruined the whole point—but I forced myself to step back and let it do its job. If you’re patching your Aktiv’s roof, Dicor 501LSW-1 EPDM Self-Leveling Lap Sealant, 10.3 oz – White is the part that actually prevents the next emergency call.

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